The _Keep_ is a rounded shell of masonry, about 67 feet by 59 feet in the internal cross diameter, and with walls, 8 feet to 10 feet thick, and 20 feet high to the rampart-walk. It is built of rubble masonry, the material chiefly Sussex stone and chalk, but faced outside with small ashlar blocks of Caen stone, close jointed, and with flat pilasters which die into the wall below the base of the parapet. The entrance was by a rather large full-centred doorway to the south-east, probably reached, as at Tickhill and Lincoln, by a flight of steps up the mound, with a timber bridge at their base, as at York. This doorway is not shown to visitors, but Tierney’s drawing represents it as having a bold chevron moulding alternating with rolls or rounds, common to the head and the jambs, and unbroken by imposts. Within, it opened into a recess, also full centred, and having its angle replaced by a bold roll. This, as at Lincoln, was the original entrance, and it seems, though walled up, not to have been otherwise altered. Near this doorway a well-stair has been formed in the wall, ascending from the ground to the ramparts. It is evidently a Decorated insertion, of the same date with a steep flight of steps and a vaulted subterraneous chamber near the centre of the area, probably a cellar belonging to the lodgings which, as at Windsor, were at one time built against the wall all round, leaving a small open court, as at Tamworth and Ledes, in the centre. That there were here such lodgings is evident from the roof corbels in the wall, and that they, as at York, had an upper floor, is shown by a late fireplace, which marks the upper level, and the back of which is formed of tiles set on edge. The parapet was about 8 feet high, and crenellated, and in each merlon was a loop, set in an arched recess. In one merlon is a small full-centred recess, evidently a garderobe. The present parapet seems a Decorated addition. The keep stands, as does the mound, on the line of the main curtain, which crosses the ditch from the gatehouse, and ascends the mound, and has a similar continuation northwards. Where the south curtain abuts upon the keep, there stands a broad irregular tower, of about 20 feet in projection by 50 feet in breadth. This contains the well and its chamber, the entrance from the curtain rampart, and an oratory. The entrance is by a vaulted passage, with a portcullis groove at the ground level of the keep. The curtain has a parapet on either face, forming a covered way, and is commanded by a sort of balcony connected with the keep. The well is of unusually large diameter, but not in use. The chamber was at the surface level of the keep, into which it opened by a full-centred doorway. There was also an upper chamber, now ruined. This position of the well, on the outside of the keep, is found elsewhere. At Wallingford it is on the slope, and at Cardiff nearly at the foot of it. The oratory, or chapel of St. Martin, is a