Mr. Harrod, some years since, at a meeting of the Archæological Society held in the Museum, exploded the theory of three circular ditches by showing from the city records that houses had always stood on the sites of the supposed outer and middle ditches; the inner vallum was the only one, and extended round the base of the hill on which the keep is erected, and is plainly traceable at the present time. It is planted with trees and shrubs, having a gravelled walk in the centre, and is enclosed with an iron palisade. The area of the upper ballium is level and comparatively high, and forms an irregular circle on the summit of the hill, surrounded by an iron railing. The great Keep situated within this area is a massive quadrangular pile, 110 feet in length from east to west, 92 feet 10 inches in breadth from north to south, and 69½ feet high to the top of the merlons of the battlements, and the walls are from 10 to 13 feet in thickness. From the basement to the top are three stories, each strengthened by small projecting buttresses, between which the walls are ornamented with semi-circular arches resting on small three-quarter columns. In the upper story the backs of some of these arcades are decorated with a kind of reticulated work, formed by the stones being laid diagonally, so that the joints resemble the meshes of a net. To give it greater richness of effect, each stone had two deeply chased lines, crossing each other parallel with the joints, so as to present the appearance of Mosaic. On the exterior of the west side are two arches which appear to have been originally intended as a deception to the enemy, giving an idea of weakness externally, where in fact was the greatest strength; for the wall is not only 13 feet in thickness in this place, but, within, it was additionally barricaded by two oblique walls which were, long ago, taken down. On the east side of the keep there is a projecting tower called Bigod’s tower, which was most probably built by Hugh Bigod, third Earl of Norfolk, who succeeded his brother as High Constable of the Castle, early in the 12th century. This tower, which was an open portal to the grand entrance of the Castle, is of a richer kind of architecture, and in the genuine Norman style, and since 1824, has been entirely restored, so as now to exhibit its pristine aspect, which is certainly different from the rest of the keep. The interior of the keep has been so greatly altered in order to adapt it to prison purposes, that the original arrangement of apartments cannot be traced.