_The Ditch._—This, by far the most formidable of the defences of the Tower, varies in breadth from 100 feet on the east to 110 feet on the north, and 120 feet on the west or City side. Along the south or river front it is only 40 feet broad, probably because on that side it was covered by the wharf, the narrow limits of which did not permit any great force of assailants to be drawn up upon it; besides which the tower covering Traitors’ Gate, and the two small postern bridges which it was convenient to have towards the wharf and river, did not allow any great breadth of ditch. The ditch was not only broad, but of great depth, so that when filled to the level of high water it was scarcely to be passed, and indeed when the water was low, the mud which accumulated there, and which made it of late years an unhealthy nuisance, must have been quite as formidable as a defence. Nevertheless, care was taken to cleanse it, and the “Liber Albus” informs us that, in the reign of Edward III., the penalty for bathing in the Tower fosse, or in the Thames near the Tower, was death! (vol. i., p. xlix.) Its exterior circuit is computed at 3,156 feet. Also, from the great height of the ground to the west and north, the counterscarp is so very high as to be in itself a considerable obstacle to crossing the ditch, although, no doubt, this was in other respects advantageous to besiegers. Besides the main bridge, the ditch was crossed by St. Thomas’s Tower and the dam between Galleyman and Iron Gate, all of which served to hold up the water. There were also the posterns of Iron Gate, Byward, and Cradle Tower. No doubt the Conqueror’s ditch, even when deepened by Longchamp, was fed by the Thames; and the water rose and fell with the tide. The intervention of the wharf and the St. Thomas’s sluice-gate were devised to make the water in the ditch independent of the tide, and thus add materially to the strength of the defences. The ditch was drained and its bottom raised and levelled by the Duke of Wellington during his constabulate. There are seen in the modern brick revetment of the counterscarp a number of walled-up arches, resembling sewer-mouths, which appear to have been intended to facilitate the mining the glacis in the event of a siege. The outwork in advance of Middle Tower, though its ditch is filled up, and its other buildings removed, is still indicated by a line of stockades, which contain the ticket-office and a small engine-house. Here stood the Lion Tower, and the Royal Menagerie; and this whole _tête-de-pont_ was further protected by a smaller tower and drawbridge of its own, shown in some of the early drawings.