But still the Turks were far from the end of their task. Long though the extent of land walls was that had to be manned, it was not difficult to protect it with a comparatively small force. A low counter-scarp enclosed a moat, over which rose the scarp surmounted by breastworks. Above this was the line of the outworks, with towers advanced here and there from their surface. Behind, and also protected by high towers, was the inner or great wall, with breast work and rampart. It was "the most perfect of Eastern fortresses,"[28] and might indeed seem impregnable. Every wall had its "military engines capable of playing on the siege-works of the beleaguering army." And as the walls "were loopholed at a stage below the battlements," the "garrison could fire not merely from the parapets but from a well protected second line of openings." While therefore it was quite possible to defend the land walls, the besieged relied for ultimate safety on being able to leave without risk the walls of the Golden Horn and the sea practically undefended. The Turkish fleet would not venture to draw near to the Marmora walls. The Golden Horn was safe with Galata on the other side--though the Genoese held aloof, through treaty probably with Mohammed--and the chain across. The Sultan had already tried to force the chain but failed. So it seemed safe:-- "Till Birnam wood shall come to Dunsinane." But the genius of the Sultan, or as one authority says, a Christian in his army, devised a scheme which at once made him the master of the city. He determined to transport his fleet overland into the Golden Horn from the Bosphorus. An extraordinary feat it was, but it was splendidly performed. A narrow canal was dug, paved, and set with rollers. The point of starting was between Top Haneh and Beshiktash, out of the range of the fort at Galata. Thence between two and three miles up the valley of Dolma Bagtché the seventy or eighty ships were drawn by night up the hill of Pera to the point where now the gardens stand just below the Hotel Bristol, and thence down the hill to the bay of Kassim Pasha where now stands the great Arsenal.[29] When the watchers on the towers of Galata and the Kentatarion by the Gate of Eugenius could see through the fogs of dawn on the morning of April 22, the great fleet was no longer before them in the Bosphorus, but behind in the Golden Horn there rode the gallant vessels with their flags flying in the breeze. The north-east wall must be reinforced. How could it be done?