The keep is built of large chalk flints grouted copiously in mortar, and cased outside with ashlar, now mostly stripped off and removed. Within, the flints are occasionally laid herring-bone fashion. The ashlar was a calcareous tufa, known in the country, and formed by the trickling of calcareous springs over moss and similar vegetation. It was much used in the earlier French castles. The later ashlar of the vaulting ribs and inserted door-cases seems to be a fine hard limestone, approaching Caen stone in appearance, and perhaps actually that material. Where the ashlar is wanting, the putlog holes are seen, placed with exceeding regularity. The joints of the original ashlar are large, those of the later fine. The new and old ashlar can readily be distinguished; but one flint wall is very much like another.
The inner ward, in length about 160 yards, and in its greatest breadth about 70 yards, is a natural chalk platform, revetted all round by a wall about 8 feet thick, which on the east side is reduced to a parapet, but on the west rises about 20 feet higher, probably its original height. In 1708 this court contained the apartments of the governor and the staff of the garrison, a well, and a chapel. These were probably of the sixteenth century or later, and have now entirely disappeared.
The enceinte wall, which girdles this inner ward, deserves attention, as most of it is of early date. Setting aside the four northern towers with their curtains, which are of later date, we have about 380 yards of curtain broken by five mural towers and three rectangular buttresses. Nearly the whole of the wall is faced with flint, with three bands of ashlar. Much of the flint is laid in herring-bone fashion, the repairs, where of brick, being of much later date. The ashlar bands are of tufa. Of the towers two, half round, are on the east front. Of these one, though probably original, has been cased with brick. The other has had an ashlar base, and the upper part, of flint, shows herring-bone work. Each is about 15 feet diameter, with walls 5 feet thick.
Between these towers are three rectangular buttresses; two of 15 feet breadth and 12 feet projection, and one about 7 feet square. The two former contain no herring-bone work, and are probably early additions, perhaps by Henry I.; the latter is original.
On the west face are now no towers, but in 1708 there were two, half round, and of small size, traces of which remain. They were, no doubt, original.
The remaining three towers capped the three salient angles of the south end, the central containing the gateway, and the others flanking it. All are one quarter engaged.
The flanking towers are alike, about 22 feet diameter and 55 feet high from the exterior base, with walls 7 feet thick. The bases are either solid or pierced by steps leading down to the galleries. There is a regular basement story, and above it a floor on the level of the inner ward. They are not vaulted and show no exterior herring-bone work, though one has a little inside. A modern summer-house has been built upon that to the south-east.
The central or gate-tower is 24 feet diameter. It is pierced at the level of the scarp by a portal which opens upon the drawbridge, the piers of which have been described; and there is a stage above this.
The portal ascends towards the keep; its details are much broken down, and little can be made of them.
Of the enceinte of the inner ward there remains to be noted the northern gatehouse. This, the original and, probably, the only entrance to the Norman fortress, though much ruined, does not appear to have been materially altered. It consists in a rectangular building, 40 feet deep by 20 feet broad, set in the centre of the curtain, with which its outer face is nearly flush. It is crossed by an outer, middle, and inner wall, each pierced by an arch of 12 feet opening, through which lies the passage. There remain the rectangular grooves of a portcullis, and a few years ago there was evidence of a second, and in the wall is herring-bone work. The arches are plain, without moulding or chamfer. The inner one is round-headed, and springs from a flat abacus, chamfered below. The joints of the ashlar work are about 1 inch wide. The curtains on either side of this gateway, though much repaired, seem to be original, and there are traces of the old round gate-towers. The pit of the drawbridge remains in front of the gate, upon the original line of fosse.