The enclosure, of about 370 acres, was in general figure a semicircle; the river forming the chord. The defences, commencing on the Thames at Blackfriars, upon the east bank of the Flete, swept in an irregular curve northward and eastward, by Ludgate, Aldersgate, Cripplegate, and the line of London Wall to where, trending eastward and southward, they took the line of Houndsditch, and appear to have abutted upon the Thames a little east of or below Billingsgate. Upon the west the Flete formed a respectable natural defence, and upon the east the line took the crest of the high ground just where it begins to subside into the low lands long occupied by St. Katherine’s Hospital, and now, more suitably, by the docks of that name. Towards the north the defence must have been wholly artificial, and is reputed to have been by a ditch which, in the later reign of King John, was deepened and made 204 feet broad, but which must have been a sufficient defence even at the time of the Conqueror. Ludgate, like the later Newgate, was placed in a re-entering angle of the wall, so that the road approaching it from the west ran for a short distance parallel to, and commanded by the ramparts. London, therefore,— “A læva muris, dextris in flumine tuta,” resembled in plan and mode of entrance those large half-round Celtic earthworks sometimes found upon the banks of a watercourse; nor is there known to have been attached to or within it anything of the character of prætorium or citadel. It is related that before the Conqueror entered London he directed a fortress to be built which should command the city. This, of course, was a temporary camp, and it was probably while he was at Westminster, or in the camp at Barking, that he studied the ground and selected as the site of his future citadel a point upon the eastern flank of the city defences, displacing for that purpose, we are told, a part of the Roman wall, including the two towers next to the Thames. Recent excavations have actually laid bare part of this wall a very few yards east of William’s tower. William was crowned in 1066, and it was from Barking, immediately after the ceremony, that he directed the actual commencement of the works, which were no doubt at first a deep ditch and strong palisade; for the keep, probably the earliest work in masonry, appears not to have been begun till twelve or fourteen years later. The new castle thus more than supplied the place of the removed works, for it could not only protect, but overawe the city, and, if necessary, cut off its trade and supplies by water. Such was the origin of this grand old fortress, the chief and central part of which gives mass and character to the group, and has from its earliest times caused the whole to pass under the name of “The Tower.”