The keep, or great tower, is the boast of Coucy, and deservedly so, being one of the finest towers in the world, and no doubt the largest and most complete single military building.
It is a plain tower, perfectly cylindrical, of excellent ashlar workmanship, 100 feet diameter at base and summit, and 200 feet high. It rises out of a paved moat, the base being about 12 feet below the level of the terre-plein, and is entered by a drawbridge from the level, all below being solid.
Including the basement, the tower contains three stories. The ground floor, on the level of the terre-plein, is entered by a drawbridge laid across the ditch, and which, when raised, covered a small square-headed portal, under a pointed arch, the entrance to a passage directly piercing the wall. The passage has an interior machicolation and a portcullis, both worked from a small chamber in the wall above, which also received the chains of the bridge. Within the portcullis was a stout door barred within, and, on the left and right, passages, one to a mural garderobe with an exterior loop, the other leading to a well-stair, which served the upper rooms and led to the ramparts.
The entrance passage leads direct into a duodecagonal chamber of about 60 feet diameter, having a recess in each floor for stores, one occupied by the entrance, one by a large well, now about 90 feet deep and formerly 200 feet, and one by a chimney.
Each pier is faced by a column, from which springs a rib, the twelve meeting in the centre at an eye, and supporting the vault. Each vaulting cell has a pointed gable, of which two are pierced for light.
The first floor is of the same figure and diameter, and vaulted in a similar manner. One of its recesses is closed by a fireplace with an oven behind it; one gives passage to a very narrow postern, the plank bridge from which drops upon the rampart of the chemise wall, and three are pierced by small windows. One of these window recesses is entered laterally by a small passage from the adjacent recess. This is of fifteenth-century work, made when the recess was walled up to serve as a separate chamber. Another recess has also a lateral passage, entering a small mural garderobe, looped from the outside. In one recess are two windows, one above the other.
The second floor, resembling the other in plan and diameter at its floor level, has a different arrangement at a height of 12 feet. Here the piers cease, and behind, between them and the outer shell of wall, is a gallery, entered by the regular well-stair, but each of the eleven other compartments of which forms a box like that of a theatre, looking down upon the central pit or floor. Two of these boxes are occupied by the detached flues of the two chimneys from below, and two are lighted by windows, which, with the central eye, form the whole and very insufficient light. In this chamber, the next below the battlements, the commander could collect and address a very numerous garrison.
The third floor, that of the ramparts, and open above, is contained within a thick and lofty parapet wall about 10 feet high, and pierced by twenty-four lancet arches and as many intermediate loops. Above these the wall is surmounted by a grand coping, which overhangs both ways about thrice the thickness of the wall, and then slopes upwards into a ridge. It was upon this ridge that were laid the roofing rafters of the bretasche gallery, which enclosed the wall inside and outside. The former was merely as a counterpoise. The latter was of two stories, and rested its main struts upon a line of forty-eight grand corbels, which remain on the exterior face of the wall at the rampart level. The flues appeared above the roof, and three large and highly-crocketed pinnacles were placed astride on the crest of the wall. The stone vault of the upper chamber was covered with lead, with occasional gutter openings outwards.
Nothing can be grander than the conception of this tower, nothing more complete than the execution of its details. All is gigantesque, as though for a race above the ordinary stature of man, and the walls within were overlaid with a fine cement, and painted with care. The design of the sculpture is bold and masculine, as becomes a military building; but all is in excellent taste, and admirably executed.
The walls of the keep are tied with chain-courses of timber, laid in mortar, in the centre of the work, as was the custom in France in the twelfth and thirteenth centuries. The timber is exposed below some of the loops. In the upper floors were embedded radiating ties, also of wood.