In the walls of the tower of Coucy can still be seen the holes which served to attach the scaffolding during construction. The spiral course which they take shows that the scaffolding, rising with the tower, formed an inclined plane of a moderate slope, up which the necessary materials could be wheeled. The advantage of a cylindrical tower from this point of view is obvious. Another structural feature is the provision of gutters for the drainage of the roof in the stonework at the back of the vault-ribs of the second floor. The absence of any effective provision for draining the centre of the roof at Conisbrough points to the probability that it was sheltered, as already explained, by a conical roof of its own.
The introduction of the cylindrical donjon in England coincides with a period at which the keep was already beginning to disappear from the castle. The principal examples, which may be attributed to the early years of the thirteenth century, are on the frontier and in the south of Wales. Chief among them is the fine tower of Pembroke (180), which was probably built by William Marshal, earl of Pembroke and Striguil, about 1200. The castle of Pembroke was of great importance, owing to its situation upon an arm of Milford haven,[214] and its command of the passage to Ireland. The keep was probably the first completed portion of the present castle, the stone-work of which, as it stands to-day, is very largely of the late twelfth and early thirteenth century.[215] It is a round tower, with a basement and three upper floors, standing just within, but not touching, the curtain which divided the inner and higher from the outer ward. The height is 75 feet; the floors were of wood, but the uppermost stage was vaulted by a dome, which still remains, rising in the centre of the tower above the rampart-walk. The stair is a vice in the west wall, from the basement to the summit: the main entrance was upon the first floor, but there is also a basement entrance, which seems to have been pierced not long after the building of the tower. The whole structure batters upwards, and the walls are slightly gathered in at each stage on the outside, a method the reverse of that pursued at Conisbrough: the masonry is roughly coursed rubble. On each of the first and second floors there is, towards the inner ward, a two-light window with pointed openings, the spandrils between which and the enclosing arch are pierced with plate tracery. The third floor was lighted by windows pierced in the dome.[216] Commanding, as it does, the whole interior of the castle, this tower is remarkably grand in situation; and its thick walls offered considerable resistance to artillery. It shows, however, no advance upon the defence of Conisbrough. The rampart-walk is narrow, and the dome in the centre prevented the employment of the roof as a platform.