By these arrangements the area was subdivided into an outer or western, an inner or eastern ward, and a central open keep. There remained, however, on the north front a three-sided space, bounded by the keep and the two curtains, and flanked by the Falconer’s and Postern towers. This space, open towards the river, is at present protected on that side by a low retaining terrace-wall and bastions of very modern date. A survey of 1567 shows this side open, and no doubt it was so originally; the river, its steep bank, the keep, the flanking towers and curtains, and no doubt palisade, being regarded as a sufficient defence. This disposition is, however, singular and very curious, and looks as though the engineer wished to attract the enemy to this the strongest and most completely flanked part, by a show of weakness by the absence of an outer _enceinte_. Economy of construction could not have been the motive, for the cross curtains, in length, would go some way towards completing the broken _enceinte_. It would seem, from existing fragments and traces of foundations, that the lines of the present _enceinte_ and keep are those of the old Norman fortress. The outline is governed very much by the disposition of the ground, and the shell keep was the approved Norman way of occupying such a knoll, whether natural, as at Durham, or artificial, as at Windsor, or, as here, probably a very slight addition to a natural knoll. A distinct ditch, now filled up, encircled the keep and protected it from its containing wards. Towards the river this ditch seems to have worked out into a steep scarp.