How to Build a Cathedral

The great cathedrals were the wonders of the medieval world -

the tallest buildings since the Pyramids.

The showpieces of medieval Christianity.

These crystal palaces were built centuries before modern architects

did the same with glass and steel.

Yet they were built at a time when most of us lived in wooden hovels,

and with little more than hammers and chisels, ropes and pulleys.

Who were the people who built them?

What drove them, and just how were they able

to build with such bold ambition?

The great cathedrals embodied the highest aspiration of the medieval age -

to represent heaven on earth.

The Medieval Consecration Service made this clear.

Oh, how awe-inspiring is this place?

It is no less than the house of the Lord. The gate of heaven.

For the medieval imagination,

just about everything in existence had symbolic value.

Just as Creation was a book written by God,

so what man did should be an image of that divine order.

For medieval thinkers, cathedrals were rich in spiritual meaning.

"The height representeth courage, the long length of the nave long-suffering,

"the breadth is Christian charity.

"As the stones of the wall would have no stability without mortar,

"so man cannot be set in the walls of the heavenly Jerusalem without love,

"which the Holy Spirit brings."

But the medieval English cathedrals that stand today

were born not of love, but war.

The Norman Conquest of 1066 brought with it an architectural revolution.

From the outset, it was clear that the new cathedrals were not only about the power of God,

they were also about the power of the invaders.

By the 1090s, almost every major settlement in the country,

from Durham in the north to Canterbury in the south,

had become a vast building site,

with over 15 cathedrals under construction.

Here in Norwich, the Normans obliterated the ancient market place, houses and churches.

Instead, they built a castle and a cathedral.

Twin pillars of the invaders' might.

Many of the Norman bishops had been close to the Conqueror.

Now they were rewarded with prestigious jobs.

The Bishop of Norwich, Herbert de Losinga, paid a small fortune for his title

and he wanted his new cathedral to reflect his new status.

It was here that Bishop Herbert built himself a mighty palace,

right next to the construction site.

From here, he could oversee building works on this enormous project.

The building of a cathedral in the 11th century

was a colossal undertaking.

All around me here would've been perhaps a couple of hundred people

working in what would've looked like a small town.

The noise and dust must have been extraordinary.

The smoke and sparks of a blacksmith's forge.

The buzz and rasp of carpenters' saws.

Stone dust, thick in the air, as the masons cut and carved and polished,

and people shouting at each other in French, English, even Latin.

The din must have been deafening.

Stage one was to lay the foundations.

We're actually under the east end of Norwich Cathedral.

This is where construction of the church itself started.

Firstly, they would have to dig foundation trenches, within which

they would put the local stone, and the local stone in Norfolk is flint.

Because they're using small flints, they would use very large quantities of burnt lime.

There's a lot of chalk in Norfolk, they can burn the chalk to make lime,

and mixing it with water and sand to create a mortar, to hold the whole wall together.

To get it as true as possible, because they're building in flint,

they'd be built between shutters, wooden panels, if you like,

which would be put in at a height of about a third of a metre.

You can see that surviving in the walls,

because the smoothness of the walls, particularly here,

can only be attributed to the fact that the panelling, the shuttering,

was there as part of that construction process.

Building usually started at the east end, where mass was celebrated.

When that was finished, the church could be consecrated and put to use.

Followed by the transepts, running north to south.

The nave and the side aisles would follow.

From foundations to finish might take less than 60 years,

or as many as 200 if the money ran out.

It's a bit of a myth that the medieval cathedrals

somehow design themselves in a great communal outburst

of religious energy, without the help of what we would call an architect.

In fact, he may not have been called an architect, but to create a structure

of this ambition, you needed a man with skill, vision and expertise.

A man who knew what he was doing.

Those men were called master masons.

They learned everything they knew on site, progressing from apprentice to stone-carver.

They travelled and made sketches of what they saw,

adapting them for their own designs to put to the bishop.

Very few medieval masons' plans or notebooks survive.

Those that do show they knew geometry.

Not the theory that we understand today, but a geometry based on the complex manipulation of squares,

circles and triangles to produce shapes and patterns

in regular proportions.

Some of these patterns can seem pretty sophisticated.

The ratio of one to the square root of two, for example,

crops up in lots of cathedrals.

It's a formula that suggests some pretty sophisticated mathematics.

In fact, it's very easy to generate using some basic geometry.

Simply draw a square,

and then draw a diagonal across it,

and the relationship between the diagonal and the side of the square

will be in the proportion of one to the square root of two.

So if you take the ground plan or footprint of a cathedral

like Norwich, for example,

the cloister is a square. Draw a diagonal across it...

and you get the length of the nave.

Create a square from that length...

and a diagonal across that square...

gives you the length of the entire church.

Right up to the high altar.

Proportion, ratio, symmetry -

to the medieval mind these were spiritual qualities.

They reflected the harmony of creation and medieval masons cared passionately about them.

Some have claimed to spot mysterious messages and codes

in the dimensions of medieval cathedrals.

The number 144, for example, which refers to the number

of those who will be saved in the Book of Revelation.

All kinds of numbers that occur in the Bible pop up from time to time

in cathedral architecture. These are mostly the ideas of churchmen.

Much more common are the basic geometrical principles

followed by the master masons -

with a square and a circle and a diagonal, you can generate

an entire cathedral.

But how much hard science did the architect know?

The knowledge of mathematics, physics and engineering that we take for granted today.

These are the things that would make a building like this stand up or fall down.

The master masons really understood stability, centre of gravity

and where that centre of gravity

would be in their piles of stones that eventually make a cathedral.

They would not have understood the concept of stress,

that is a modern concept.

It's the intensity of load, it's how much load per unit of area,

and in a cathedral like this,

that intensity of force is really comparatively low.

It's about 100 tons on every square metre.

So if you think of these columns as one square metre by one square metre, they could easily carry 100 tons.

Indeed, they could carry 1,000 tons.

Those columns in the nave are huge and they are the weightiest part of the cathedral.

Indeed, when I worked out the weight of one bay of Norwich Cathedral,

the total weight was about 1,800 tons for one bay,

of which about 1,000 tons was in the columns.

So you can see how much weight went into the columns and the walls.

Very little was in the stone vault at the top.

Only about 3% of that weight, I found, in the thin stone vault -

and this is the secret to success.

Keeping the weight light at the top, heavy at the bottom,

to bring the centre of gravity down to a lower level.

And then there's a stability element.

This arch allows these two columns to work together to form a stable whole,

like standing on two legs instead of standing on one.

You can repeat the exercise by putting more weight on the outside than on the inside.

This makes this extremely steady and strong.

I think it's possible to devise a set of rules

by which the stonemasons would have worked.

For example, the slenderness ratio of a column.

That is the height of the column divided by its width.

For most columns, if you keep that below ten, that's fine.

Again, the arches and the vaults were proportioned with the set of rules.

The thickness of a vault is roughly 1/20th of its span.

Here at Norwich, with a span of 10m, the vault is about 200mm thick.

And that works fine.

So these are just some of the rules that the stonemasons

would've passed on through the family line of father to son.

Worshipping in churches built partly by rule-of-thumb

wasn't without the odd anxiety.

The congregation of Beverley Minster in Yorkshire, around 1213,

grew uneasy at the new building works in their great church.

"The craftsmen who were in charge of the work were concerned with beauty rather than strength.

"They led the new work into the old ingeniously, but not firmly.

"In the manner of those who sew new cloth into old.

"At last, it came about, that from fear, a great part of the clergy and people refused to enter the church."

And there were occasional disasters.

The central tower at Winchester Cathedral fell in 1107.

One of the west towers at Gloucester collapsed in the later 12th century.

At Norwich, the spire blew down in 1362.

Many problems came from poor foundations

which caused the stones to slowly shift and crack.

Most people now think that the cathedrals were actually heavily over-engineered.

You could take quite a lot of stonework away from these buildings without them falling down.

In an upstairs room at York Minster you can get a tantalising feel

of the early stages of a master mason's work.

This is a very, very special place.

These are the living quarters and the workshop of one of the top master masons in the country.

The man who designed York Minster.

Here, he might have come up with some of his best design ideas, perhaps

sitting on his private toilet, which still exists over there.

It is not just a place for thinking, sleeping and eating.

Here is his drawing board.

His drawing board is the floor, and scribed on it

are the individual lines, individual pieces of stone.

All around us are similar patterns which over the years might have become quite a headache to work with.

So every now and again, he had to put down a new layer of plaster.

And to make it really firm, perhaps dozens of people,

perhaps some of them children, would have come up here and walked around in socked feet and we can still see

the heels of medieval masons on this floor it to this day.

Making sure it's as firm as possible for the next series of designs to be worked in.

There's a feel of the Mary Celeste about these marks.

As if the master mason had just gone off for lunch.

And the windows built from this design are still there,

just a few feet away, at the east end of the Minster.

The next task for the master mason was to choose the stone.

Up to 80,000 tons of it.

Dragged to the site by ox and cart, or floated down-river on barges from the quarries.

This quarry supplied stone for Lincoln Cathedral.

It was a matter of bars, wedges, horses, carts.

Quite a labour-intensive process.

-They're naturally bedded, you can just see one going through there.

-I see, that flat line.

And there will be another one down here and down here, so you've got three different beds.

They knew that, medieval masons knew the best stone, the best stone

was the silver bed which was the top one.

The silver bed was top-quality carving for your carvings, your caps, angels, grotesques.

The next one was the lower silver which was still nice to work but

a little bit softer, and then the red bed.

They all had their individual usage.

These are going to end up as beautiful carvings.

You've got to have some pretty strict quality control, I imagine?

You have. Even from the very early stages of quarrying the block,

if you see quite a few natural cracks in that, it is put to a waste pile.

The quarrymen were quite good at their job

because they wouldn't go to the trouble of getting it out if it wasn't any good anyway.

The final say was the master mason.

It was hard labour and especially all that work you went into to get them blocks out,

then realise it had a crack in it, it must have been so frustrating.

So I do take my hat off to them.

The master mason then hired a team of stonemasons

to work the roughly cut blocks.

They followed templates made from his designs.

The templates applied to the outside of a block of stone.

They guide the masons,

show the masons what shape the stone should be.

If they follow the instructions on the templates

and they follow the shape of it,

they will end up with a block that's cut to the correct size.

You may need several templates for one stone, depending on its complexity.

If it's a very complex piece of tracery, for instance, or a springing stone, you would

need a section mould which would go on the side to give you the profile.

A bed mould that would go to the top

and apply to the bottom, probably two different moulds for that.

You would have to have a face mould, which would show the shape of the stone when viewed from the front

and any lines of moulding that are on there.

It's almost like dressmaking. It's the same principle.

So, this is the final destination of a piece of stone?

Yes, this one has come from the quarry, it's been worked.

We've cut a space for it to go in here.

We'll just put a little bed of mortar on here.

This is just a mixture of sand and lime.

Medieval builders used a lime mortar which sets less rigidly than modern cement.

So the stones could shift and settle over centuries.

Here I've got Lewis pins.

Which are very similar to ones that would have been used in medieval times. I put those in the hole.

As I pull on them they'll open up, like so, and they'll grab hold of the stone for us.

Yes. We can get it into the edge and the stone below

looks like it's from some previous restoration. But the one above

has been there for probably about 600 years.

Somebody presumably has had to come up here and do a very detailed measurement of that

and create a template and somebody down the mason's yard

has used that template to come up with this lovely, clean, smooth bit of moulding, here.

In cathedral building, ancient craft met modern assembly line.

Each stone individually designed and carved, then repeated,

to create symmetry and pattern.

At Durham, the master mason prescribed

the precise size and quantity of blocks he would need to build the church.

The great zig-zag and diamond patterns on the columns were created

from five standardised designs.

850 blocks of one size, 600 of another, 230 of a third, and so on.

By arranging these in different ways

all the patterns on the columns can be generated.

It's as if the church was a vast jigsaw.

The building of the cathedral tested the skills and ingenuity of the cathedral builders to the limit.

Yet the ideas and methods that they used had barely changed since Roman times.

In fact, the style they built in,

in the early period, is called Romanesque.

It's based on the kind of arch that Romans used, the round arch.

The shape was always based on a circle.

Often a full semi-circle,

sometimes a section of one, or with the curve slightly distorted.

It was simple, but limited.

Arches in the style known as Romanesque can be sturdy, even chunky.

A whole building made of these arches has a muscular, powerful effect.

But in the middle of the 12th century came an idea that would change all this.

A revolution in design and construction

which would raise cathedral building to new levels of sophistication.

It was a style that would become known as Gothic.

It was born in France, at Saint-Denis, near Paris.

Here, the visionary Abbot Suger rebuilt part of the abbey church,

burial place of the French kings.

He was using a new kind of arch.

Not a round arch, but a pointed one.

But his aims went beyond this.

For Abbot Suger had a vision.

He wasn't simply after something bigger and grander.

He had a theological rationale for his new design.

It was a theology of light.

For medieval thinkers, light had a profoundly religious meaning.

The embodiment of spirit, one writer called it.

"The work should brighten our minds

"so they travel through the light to the true light of Christ.

"In seeing this light, the dull mind is resurrected from darkness."

Abbot Suger and his unnamed master mason

had glimpsed the possibilities of the pointed arch.

It's both stronger and more flexible than a round one.

Both the shape and width of a pointed arch can be varied in more ways.

And the weight of a building naturally moves downwards

and outwards in a shallow curve

closer to that of a pointed than a round arch.

This simple difference was to change

the entire way cathedrals were conceived, designed, and built.

The second Sunday in June 1144

may have had a greater effect on architecture than any other day in history.

At the dedication service, bishops from all over Europe gathered at Saint-Denis

to witness a building designed to evoke the experience of heaven

more completely than anything before it.

The bishops were awestruck.

They had seen the future.

There was one dissenting voice - the monk Bernard of Clairvaux

saw in such splendour nothing but worldly distraction.

"Oh, vanity of vanities, but more folly than vanity,

"every part of the church shines but the poor man is hungry.

"The walls are clothed in in gold while the children of the Church remained naked."

But such asceticism was out of step with what the mighty bishops wanted.

England was the first country outside France to take up the new style.

Lords and bishops had estates in France and French was the language of court.

When fire devastated the east end of Canterbury Cathedral, the most important church in the country,

the monks here saw their chance.

The man in the 1170s chosen to be the new master mason

was, significantly, a man who had worked for years in France.

His name was William of Sens.

His task - to create one of England's first Gothic cathedrals.

The Gothic style started a push to build higher.

It would place new demands on the unsung heroes of cathedral building,

the carpenters.

They built scaffolding and the timber frames

around which entire sections of the cathedral were built,

including the arches themselves.

We start off with a wooden centring.

Which we put in here.

Then we just start off down the bottom of the arch,

building the stones up either side.

They're built on little pads,

which space it out nicely to give it a nice joint.

And then, once you've got to the top, you put in the keystones

and they'll wedge it all into place so that it won't move.

After that, we pour lead into Y-shaped grooves that run down

in between each of the stones, that makes it even more solid.

Then we finish off by putting mortar into the joints, pointing it up.

So putting up this wall is the final stage of the process?

Yes. Once the wall is up around it,

we can take the centering out and the arch will support itself.

And putting up arches like this is the basic building block of creating a cathedral?

Very much so, you can create very large spaces inside the building

with much bigger arches than this.

It's the same technique that the medieval masons would have used.

Massive quantities of wood were required to build the cathedrals.

Almost 1,500 trees had to be cut down to build Salisbury Cathedral.

Often, the wood wasn't available locally

and had to be sourced from abroad.

Pine from the Baltic, oak from Ireland,

entire forests are locked up in the walls of the cathedrals.

And to get the great wooden beams and pieces of stone

up from ground level,

masons and carpenters used ropes, winches, pulleys, ladders

and the Big Bertha of them all.

This is the windlass that the masons would have used

to lift enormous blocks of stone

to the upper levels of their construction site.

By holding one of these pegs and turning this great wooden wheel,

a mason could lift more than ten times his own weight.

As the wheel turned, the timber beam turned

and down here,

you can see the gouge marks made by the ropes as they lifted the stones.

It was a precarious and dangerous business.

This strange little carving

shows a mason falling from the upper levels of the building.

We don't know exactly what happened to him,

but we do know what happened to William of Sens at Canterbury.

Some years into the project,

he was working 50ft above the floor, up among the roof timbers.

-MAN NARRATING:

-"Suddenly, the beams broke under his feet.

"He fell to the ground, stones and timbers accompanying his fall.

"Sorely bruised by the blows of the beams and the stones,

"he was rendered helpless alike to himself and for the work."

William was paralysed and had to return to France.

But Gothic was here to stay.

No-one had seen anything like this before.

The lavish use of polished stone,

the vast expanses of jewel-like glass.

This new East End was four metres higher than its predecessor

and a third longer.

But what was really revolutionary about it

was the use it made of the pointed arch.

This gave the interior a kind of tense harmony

that was novel, modern, almost shocking.

Canterbury and the cathedrals that followed it

seemed to float heavenwards, infused with colour and light.

Within a generation of Gothic appearing in Britain,

it had developed into an innovative native style.

Where the French built high and austere,

in England, Gothic would turn into something rich and ornate.

Often dramatic, sometimes fantastical.

At the cathedral in Wells,

the Gothic style came to be used in a wholly new way.

Every year on Palm Sunday,

the cathedral building itself became a stage set for religious ritual.

Dressed in their most magnificently embroidered clothes,

the clergy formed a great procession

which would head towards the grand west front.

Clouds of incense surrounded them.

They were about to take part in a piece of sacred theatre,

re-enacting Christ's entry into Jerusalem.

"Lift up your heads, o ye gates, so the king of glory can come in,"

they chanted in Latin.

It was the architecture that answered back.

For behind the west front, with its 176 life-sized painted statues,

the builders had created a space for a hidden choir.

The statues seemed to sing out the response across the cathedral close.

For a few brief moments, architecture, sculpture

and a kind of sacred theatre had fused

and this church in the English West Country

had become Jerusalem itself.

Medieval cathedrals revelled in these kinds of dramatic effects.

Colour was everywhere.

Walls were whitewashed and painted with patterns and sacred scenes.

Carvings brought to life with paint.

Columns polished to look like precious stones.

And perhaps the key innovation of Gothic,

huge windows to allow the light of God to pour in.

Filtered through richly stained glass,

stage lighting for the theatre of Gothic.

The spectacular east window of York Minster was designed

by John Thornton of Coventry in the 15th century.

It tells the story of the Creation and the end of the world.

Today, it's under repair.

The conservators using much the same technique as the medieval glaziers.

The starting point is a cartoon.

The artist not only needs to draw in the detail on a plan or a cartoon,

but also needs to tell the glass cutter where to cut the glass.

In the medieval times, when this panel was being made,

the dark lines would have indicated where to cut.

So you need to cut glass first.

The glass could be brought in from the Continent.

We think France or the Low Countries,

because they were making the best quality of glass at that time.

The impurities in the materials that make up the glass

give it a beautiful tint of greenish or yellowish,

so the border pieces are tinted glass

but if you look inside, you can see different colour glasses, red, blue.

That would be brought in as blue glass.

At the molten stage, oxides would be added,

in this case cobalt would be added to the melting pot

to make that glass blue.

The rough shapes would have been laid out and then finely grozed

by just nibbling away at the edges with a bar.

Once the glass was laid on the sheet, the painter could begin the work.

Then the glass was joined together using strips of lead.

When it was put in place, it was perhaps the largest stained-glass window in Europe.

Gothic and stained glass made a perfect marriage.

Cathedral builders vyed to make ever-larger windows, to make wall spaces ever smaller.

Some buildings became mere skeletons of stone.

But these structures, with their great roofs and ceilings,

pushed outwards as well as downwards.

They needed strong support.

The masons came up with a new kind of structure.

The flying buttress takes the weight of the building out and down,

leaving its walls free for all kinds of possibilities.

Windows could get bigger, the structure itself could get higher.

And arches of all kinds could be more delicately decorated.

The flying buttress siphoned off the weight from the walls of the nave,

leapfrogging the side aisles as it did so.

Where practical necessity met aesthetic adventure

came a strange, fantastic beauty.

For more than 300 years, from the late 12th century to the early 16th,

cathedral architecture in England enjoyed a golden age.

MAN NARRATING: "Windows make the upper parts of the cathedral bright,

"with their vivid and various colours imitating the rainbow.

"Hard without, but like a honeycomb within.

"It seems to be a thing not of art but of nature,

"holding people's minds in suspense as they wonder.

"Soaring and lofty, clear and resplendent."

The last part of a cathedral to be built

was the stone ceiling under the roof level known as a vault.

Here, at Gloucester, you can see how as masons grew bolder,

their vaults became ever more ambitious.

The great breakthrough came with the invention of the rib vault.

These small, diagonal arches

made it possible to build vaults there were bigger and higher.

Though at first they were built to strengthen the vault

and hide ugly joins, in the 13th century,

masons began to exploit the purely decorative effects of these ribs.

They add these diagonal ribs, which are called tiercerons,

that are more decorative, simply about making patterns.

Soon you are in what we call the decoratived style.

You're heading through the 14th century,

and people start to get really carried away, they add little

short ribs which are just there

to make three-dimensional patterns on a curved, stone roof.

Above the Lady Chapel at Gloucester Cathedral,

you can really get a sense of how such elaborate vaults were constructed.

50ft above the ground,

the master mason shows me the reverse side of the vault.

OK, so we are on top of the Lady Chapel here.

And we're going to climb

over this vault, so we will have a good view

on the structure.

You have a volume of space to cover with a particular ceiling,

which in that case is a Gothic vault.

So first you fit that space with a scaffolding frame, made of wood.

You position the wood arches,

the wood centring,

which are just underneath the stone ribs.

It will support all the individual stones constituting the stone ribs.

As you can see here, you have got the main arch, the main ribs,

and they're all good solid stone.

But you see that funny-looking stone inbetween?

That's tufa. It's something which is very, very light

and spongy, and very light.

It's a material, which is probably less than

half the weight of normal stone.

You can extract it, quarry it in the field.

When it is fresh, it is extraordinarily soft.

Then exposed to the air, it will harden very quickly.

You can see all the two marks, the double-edged axe marks,

which was the primary tool of the medieval mason

to produce quickly a unit of stone.

So it was done in there, and he started at that corner and came down.

They are the typical radiating marks

of the double-edged axe, going down that way.

So these large, square stone,

they are at the junction of all the ribs, and the carving of the bust

is to hide this junction, and to make a use of this junction,

and it also it was a useful device to hide possible inaccuracies

where it was not quite meeting,

which was occuring from time to time.

In the cloisters at Gloucester,

mastery in stone and aesthetic ambition reach a climax.

The fan vault must rank as one of the marvels of the English Gothic style.

These trumpet-like cones covered in carved patterns

were unlike anything seen before.

The fan vault dispenses with ribs altogether.

It is just a skin of carved stone a few inches thick,

like an eggshell.

Yet the effect has all the delicacy of lace.

It was a kind of Eureka moment for the medieval masons.

Instead of ribs, they simply build a smooth, curved surface,

spreading out evenly in all directions like an inverted dome.

The surface was covered with repeated patterns

reflecting those that ran over the walls and windows below.

Again, no maths or physics were needed, just what worked.

The idea is inspired,

but the engineering not much more complex than constructing an igloo.

The fan vault was one of the last, great innovations

in three centuries of Gothic.

Growing technical sophistication

fed a demand for more elaborate architecture.

Patrons wanted more of everything; more variety, richer patterns,

more fantastic designs, and more breathtaking architectural effects.

All over England,

the great cathedrals were rebuilt in the rich, new style.

From the soaring nave of Canterbury,

to the elegant choir at Salisbury,

and the luminous stained glass of York.

Sumptuous Lady chapels were added,

borne of a passionate new attachment to the Virgin Mary.

But there was a price to be paid for such ambition.

At Lincoln, in the the 1190s,

the cathedral was rebuilt in an elaborately ornamented Gothic style.

So carried away was the master mason, he neglected to build columns

strong enough to hold up the tower he was about to build.

The result:

before the tower was even finished, it came crashing to the ground.

Yet medieval masons were good at turning disaster into triumph.

The central tower of Wells Cathedral

also showed signs of imminent collapse in the 14th century.

The masons came up with an astonishing solution.

Gigantic scissor arches that braced the arches supporting the tower.

This bizarre, but graceful edition stabilised the entire structure.

But the most innovative response to calamity

happened 200 miles to the east.

On 12th February, in 1322,

the monks of Ely Cathedral had just sung Matins.

MAN NARRATING: Suddenly and swiftly

the bell tower crashed down upon the choir

with such a thunderous noise, one might think an earthquake had occurred.

The fall devastated the central part of the cathedral

and plunged the senior monk Alan of Walsingham into despair.

Alan of Walsingham would have been faced by an enormous pile of rubble.

The first thing to do was simply to clear that,

and perhaps as he was doing it an idea might have begun

to take shape of quite a bold solution

to the problem of how to replace a fallen Norman tower.

The church, like all other churches, had four great arms;

east and west, and north and south,

and an entire section at each of these four arms was demolished

making them shorter and this space much bigger.

And then he joined these four arms with four angled walls,

creating an enormous octagonal space about 23 metres wide.

Having laid this out,

the masons had to dig down two or three metres, find new foundations

and then they started to build upwards.

And they went up and up, 20 or 30 metres into the sky,

but they weren't really building walls, but a kind of skin,

a skin pierced by enormous arches,

the kind of thing that is only possible with Gothic architecture.

And when he'd finished, he had an octagon of stone.

And the question was, what to put on top of it.

To roof this enormous, octagonal space,

Alan of Walsingham worked with one of the king's master carpenters.

The the plan was to do it all in wood.

Timbers had to be sourced.

Some of these are 12 metres long,

and they are still covered with axe marks made by medieval workers.

Between them, they devised an ingenious wooden framework,

a colossal structure

which literally sits on top of the stone sheath of walls they had already created,

and rises up from it, going higher and higher,

so that it suspends a great wooden vault

which, from underneath, looks like it's almost weightless.

Once again, medieval craftsmen had turned a disaster

into a colossal piece of architectural theatre.

Of all the spine-tingling moments of the medieval cathedrals,

this has to be the tops.

All the lines of this enormous octagonal space

rise up and converge.

And just when you think they're going to join,

they stop and go up again into a cage of coloured light.

And right at the top, at the heart of it all,

is a beautiful carving of Christ in Majesty.

The octagon at Ely Cathedral survives from the 1320s,

a decade of architectural brilliance

whose masons should be as famous as Turner or Shakespeare.

But this was not an age of ego.

The reputation of these men lies buried in the stonework

and soaring vaults of these amazing buildings.

The cathedrals today are a journey through time, not a moment in time.

Each one has been rebuilt or extended several times over the centuries.

Not a single one is as it was when it was first built.

To walk through an English cathedral

is to walk through the history of England and its architecture.

Here at Ely, for example, the great round arches of the nave,

with their massive pillars, date back to the conquering Normans.

Walk along, and you enter the 13th century and the pointed arches of Gothic.

Further still, and you get to the very end of the medieval era

with its passion for carving, that looks less like stone than crystallised foam.

But the time would come when the great cathedrals

no longer exerted the same fascination.

The intellectual and religious passion

that had fired the medieval builders was changing.

Wealthy patrons put their money into private chapels,

rather than great churches.

And in the 1530s, the religious world of England was turned upside down.

Protestants and Puritans put hammers and chisels to a very different use.

But the splendour of the great cathedrals, the commitment

and skill of the people who created them, these remain.

Reminders of a glorious ambition;

to realise the vision of the Book of Revelation,

to build heaven on earth.

-MAN NARRATING:

-"The New Jerusalem, pure gold like unto clear glass,

"garnished with precious stones

"needed neither the sun nor the moon to shine in it.

"For the glory of God did lighten it.'

Subtitles by Red Bee Media