The design of this castle ditch is peculiar. The more obvious plan would have been to place the walls upon the edge of the hill, and scarp its sides down to the valley with such steepness as suited the ground. Instead of this, the upper 40 feet of the hill, being chalk rock, was scarped vertically, and then faced or revetted by a wall, upon which was placed the _enceinte_ wall of the inner ward. At the foot of the revetment was then excavated the ditch just described, the material being thrown outwards so as to form an artificial scarp, which thus became a sort of advanced banquette or earthwork beyond the ditch, representing the crest of the glacis, and capable of being held by a line of soldiers, but which, when taken, was too exposed and too narrow to allow of its being held, or of cover being constructed upon it. This banquette was at a rather lower level than the foot of the opposite wall. Such an arrangement is found in other castles in Normandy, and notably, as pointed out by M. Deville, at Molineaux, De Longueville, Bec de Montagne, and in the later work of Château-Gaillard, where, however, it is less marked. The keep is rectangular, about 80 feet north and south by 70 feet east and west, and at present about 60 feet high. It stands in the south-west corner of the inner ward, close to the _enceinte_ wall, of which its south-west angle forms a part. Its walls at the ground level are about 13 feet thick. It presents three buttresses on the north face and two on the south—the third being a sort of cap thickening and enveloping the south-east angle. These are of the unusual breadth and projection of 9 feet, and they rise to the present, which cannot be above 10 feet below the original summit. At the north-east angle the adjacent buttresses are set square, leaving the angle free. The south face is plain, or nearly so, excepting the cap at its south-east angle, which extends southwards, and is connected with the adjacent _enceinte_ wall. The west face is plain, outside of which was the entrance. This is composed of a flight of steps, beginning upon the north face, passing by a doorway through its most westerly buttress, and which then, turning, is continued along the west face, until at its south end it lands in the usual square appendage or forebuilding common in these keeps. This staircase was guarded by an exterior wall, and had gateways at its foot and its summit. It was covered over, as appears from marks upon the wall; and above it was no doubt the usual platform for defence. In the basement of the forebuilding, which forms also part of the _enceinte_ wall, was a vaulted chamber opening into the basement of the keep, as at Rochester, either a store or a prison. The landing story was barrel-vaulted, having at one end a loop towards the field, and at the other a door in the wall of the keep.