The mound on the eastern face, measured from the scarp of the ditch, is about 30 feet high, but on the western side, where it rises from a lower level, it is about 50 feet, or 92 feet above the river. The mound and the ditch evidently supported and protected the dwelling of the English lord, and it is probable that upon the platform below, where the Norman king afterwards placed his hall and offices, were lodged the serfs and dependents of the earlier household. Judging from the close analogy of Leicester, Tamworth, Tutbury, and other earthworks of known date, the earthworks of Guildford may, with great probability, be referred to the earlier part of the tenth century. The keep, a rectangular structure, covers the eastern slope of the mound, but is placed a little to the south of its central line, so as to allow of a gateway (now gone) at its north-east angle, and a passage up the mound outside the north wall. The east or lower wall rests on the undisturbed ground, a little above the level of the scarp of the ditch, and the west or upper wall upon the edge of the level summit of the mound, nearly the whole of which thus extends undisturbed to the west and north-west of the building. The difference in level of the base of the two faces of the keep is about 15 feet. It is exceedingly rare to find a rectangular keep placed upon an artificial mound. Guildford, Christchurch, and Clun are the only recorded examples. The keep stands nearly by the points of the compass, measuring 46 feet north and south, by 52 feet east and west. The wall is perfect to the base of the parapet, a height on the west front of about 63 feet. The masonry of the lower side contains more ashlar, and is of better quality than the rest, to prevent the structure from slipping. Of the depth of the foundations nothing is known, but the thickness of the wall—at least 11 feet at the visible base—would serve to distribute the load, and chalk, even when made ground, does not make a bad foundation. There was, no doubt, a risk in placing so heavy a building upon an artificial hill, even though a couple of centuries old, but the result has justified the means employed, for there is not a crack or mark of settlement in the whole edifice. Grose represents some half-buried arches on the south side, not now visible, but which, if they ever existed, which is more than doubtful, might indicate that parts of the building rested on piers, carried down to the solid ground. However, enough of the wall is bared to show that this is not the case. What Grose took for an arch was probably a low course of inclined or half-herring-bone masonry. Others have described an opening on this side supposed to lead to a sub-basement vault which there is no reason for supposing to exist. The machicolations cited in evidence as defending this fabulous doorway are the vents of a garderobe in the upper story.