COUCY

Nothing like it was built before, and nothing like it has been built since.

On a chalk spur above the Ailette valley in Picardy, a man who was not a king, a prince, a duke or a count put up a round tower 55 metres tall and 31 across, with walls seven and a half metres thick. It was not the cleverest castle in France and it was never the strongest position. It was simply the largest statement in stone that a private person has ever made, and it stood for 692 years before anyone managed to bring it down — which took an army, a railway, and twenty-eight tonnes of high explosive.

Coucy was not one fortification but three nested ones, and you had to beat each in turn.

1. The town. Coucy-le-Château is a walled ville in its own right, ringed with towers and entered by three gates — the Porte de Laon, the Porte de Soissons and the Porte de Chauny. Taking the castle meant first taking a town.
2. The castle. A roughly quadrangular enceinte closing off the tip of the spur, with four cylindrical corner towers, each around 20 m across and 35 m high. Any one of them would have been the proudest tower in a lesser castle.
3. The donjon. Set at the most exposed angle, where an attacker would come, and cut off from the rest of the castle by its own ditch and its own ring wall — the chemise. If the castle fell, the keep did not. It was a fortress inside a fortress, provisioned and defensible on its own.

There had been something on this rock long before that. An early fortress here is credited to Hervé, Archbishop of Reims, around 920, thrown up against Norse raiding. The site kept being fortified because the geology invited it: a narrow promontory with steep falls on three sides, so an attacker had exactly one approach and the defenders knew precisely where to point everything.

The engineering headline is that middle number against the fifth one. Each floor is a circular room twenty metres across with nothing holding up the middle. The ribbed vaults spring from shafts set around the wall and meet overhead, and the load is carried outward into a wall thick enough to have rooms, stairs and passages tunnelled inside it. A spiral stair runs up through the masonry; the garrison moved between floors without ever crossing open ground.

Each vault was pierced by a central oculus, an eye open to the floor above. It let light down and let loads be hauled up the full height of the tower without a crane outside — useful in peace, essential in a siege.

The sires de Coucy were barons. Not counts, not dukes — barons, holding one exceptionally good lordship on the road between Paris and the north. They behaved like a great deal more than that, and the tower is the argument.

The house is remembered for a boast that survives as its device: after listing every title they did not hold, it finishes "I am the lord of Coucy." It is almost certainly a later invention — the wording is not attested in Enguerrand III's own century — but it stuck because it is exactly right about the building.

Enguerrand III (c. 1182–1242), called the Great, is the one who built what you see. He fought in the Albigensian Crusade, quarrelled with the crown, and during the regency of Blanche of Castile was talked about — by his enemies, at least — as a man with designs on the throne itself. He died in 1242 after falling on his own sword when his horse threw him.

The last lord of Coucy is the one historians keep coming back to, because his life happens to touch nearly everything that made the 14th century terrible.

Enguerrand VII (1340–1397) was sent to England as a hostage for the French king's ransom after Poitiers. He spent his captivity so agreeably that Edward III married him to his daughter Isabella, released him, and made him Earl of Bedford. When the war restarted he renounced the English title rather than fight his own king — and then spent decades campaigning across Europe anyway. He died in 1397, a prisoner in Bursa, after the crusading army he had joined was destroyed by the Ottomans at Nicopolis in 1396.

Because his life is so unusually well documented and so unusually well travelled, Barbara Tuchman used him as the thread through A Distant Mirror (1978) — her history of the calamitous 14th century. If you want the plague, the schism, the free companies and the chivalric disaster of Nicopolis in one narrative, that is the book, and Coucy is its home base.

Not everything here was military. The castle's great hall held the Salle des Preuses — nine over-life-size figures of heroines set around the room, the female answer to the Nine Worthies who were the standard medieval set of exemplary heroes. Commissioning nine preuses rather than nine preux was a deliberate and rather pointed piece of decoration, associated with Enguerrand VII's household.

They are gone. Fragments were recovered and a few heads survive in museum collections, which is the usual arithmetic at Coucy: a description, a drawing, and a piece of the original.

Coucy passed out of the family in 1400, bought by Louis, Duke of Orléans, and from there into the hands of the crown. A castle that strong in private hands was a standing problem for a centralising monarchy, and during the Fronde the problem was solved the usual way: in 1652 Cardinal Mazarin ordered it slighted.

They mined it. The charges cracked the donjon and opened fissures that ran the height of the tower — and the tower stayed up. For the next two centuries it stood cracked, roofless and enormous, which is the state in which it became famous.

The 19th century fell hard for Coucy. Eugène Viollet-le-Duc — the restorer of Carcassonne and Notre-Dame — surveyed and consolidated it from the 1850s, and used it repeatedly in his Dictionnaire raisonné as the worked example of what a great donjon was and how it functioned. The castle was classified a monument historique in 1862. Iron ties were put in, the cracks were watched, and the tower was made safe enough to visit.

That work is why we know as much as we do. When the tower came down, it came down having been measured, drawn and photographed more thoroughly than almost any other medieval building in France.

A ring. The donjon survives as a circular stump of that colossal wall, standing a few metres high, with the interior full of its own collapsed mass — and because the wall was 7.5 m thick, the stump alone is wider than most complete castle towers. The corner towers are the same: bases, and heaps. The town gates and long runs of the town wall came through, being of no interest to demolition parties.

It is open to visitors, in the care of the Centre des monuments nationaux. You walk around the inside of a circle you cannot quite believe was a room.

• Barbara Tuchman, A Distant Mirror: The Calamitous 14th Century (1978) — the life of Enguerrand VII as a way into the whole century.
• Viollet-le-Duc, Dictionnaire raisonné de l'architecture française — the 19th-century survey drawings, and the reason the building is so well understood. Long out of copyright and freely readable.
• The Centre des monuments nationaux site for the castle as it stands, and for visiting.

• A cutaway of the three halls at true scale with a figure drawn in — the numbers do not land until you can see a person against them.
• A before/after slider from the pre-1917 photographs, which exist in quantity.
• The whole spur in plan: town, castle and donjon, so the three-walls idea is something you look at rather than read.