The _Keep_ stands upon rather the highest part of the enclosure. It is rectangular, 43 feet east and west, by 51 feet north and south, and stands upon the curtain, with which its west and south faces are continuous. As it does not quite cover the whole end of the ward, this is closed by a low curtain, 17 feet long and 3 feet thick, which extends from the keep to the north-west angle of the ward. The keep is composed of a basement and three floors, and is about 60 feet high. The parapet is gone. It has a plinth only on the two exterior faces, where the ground is low, and there are two sets-off which indicate the level of the second and upper floors. At the end of each face is a pilaster, 7 feet broad, and of 6 inches projection, and those adjacent meet and form a solid angle. These pilasters rose clear of the wall to form angle turrets, of which parts remain. From the upper set-off, on the north and south faces, rises an intermediate pilaster, 3 feet broad. The walls at the base are 10 feet thick, and, at the top, 6 feet. The basement is at present nearly filled up with earth and rubbish, concealing much of the east side. Part of the south-east angle fell in in 1792, and obscures the details of the main entrance. The _Basement_ is at present entered by a plain round-headed doorway of 4 feet 7 inches opening, in the north wall near its east end. This has a rebate for an inner door, but no groove for a portcullis. The outer jambs are broken away. It is pretty clear, from a comparison of its ring-stones with the original arches above, that this entrance is an insertion, probably of the time of James I. or Charles I. In the same side, near the doorway, is a very peculiar air-hole, formed of two loops, 2 feet apart, which converge to a single exterior loop. There are similar loops, of much later date, at Caernarvon. Possibly the basement was divided into two chambers, and one loop opened from each; but there is no trace of any partition. In the west wall there was probably another loop, now converted into a window of 2 feet opening, square topped, set in a bold splayed recess, evidently an insertion of a period when security was no longer the first consideration. The window opens in the line of the plinth, the set-off of which is carried over its head as a square hood-moulding. The south wall was blank, and so, probably, was that to the east. The basement was 13 feet or 14 feet high, with a flat timber ceiling. There certainly were no mural chambers, and no staircase in it. It was probably entered from above by a trapdoor and ladder, and used as a store.