In the autumn of 1452, while Mahomet was finishing the castle on the Bosporus, a Hungarian or Wallachian cannon founder named Urban, who had offered his services to the emperor and had been engaged by him, was induced by higher pay to go over to the enemy. He would have been content, says Ducas, with a quarter of the pay he received from Mahomet.[223] After learning from him what he could do, the Turks commissioned him to make as powerful a gun as he could cast. Urban declared that if the walls were as strong as those of Babylon he could destroy them. At the end of three months he had succeeded in making a cannon which remained for many years the wonder of the city and even of Europe, and marks an epoch in the continually increasing power of guns. The casting was completed at Adrianople.[224] In January it was started on its journey to the capital. Sixty oxen were employed to drag it, while two hundred men marched alongside the wagon on which it was placed to keep it in position. Two hundred labourers preceded it to level the roads and to strengthen the bridges. By the end of March[225] it was brought within five miles of the city. But, though the fame of this monster gun has overshadowed all the rest, we shall see that it was only one amongst many.[226] [Sidenote: Turkish fleet.] Above all, says Critobulus, Mahomet had given special attention to his fleet, ‘because he considered that for the siege the fleet would be of more use than even his army.’[227] He built many new triremes and repaired his old ones. A number of long boats, some of them decked over, and swift vessels propelled by from twenty to fifty oarsmen were also ready. No expense had been spared. The crews of his fleet were gathered from all the shores of Asia Minor and the Archipelago. He selected with great care the pilots, the men who should give the time to the oarsmen and the captains. At the beginning of April, his fleet was ready to leave Gallipoli, which had been the place of rendezvous. Baltoglu, a Bulgarian renegade, was placed in command. A flotilla of a hundred and forty sailing ships started for the Bosporus.[228] Of these, twelve were fully armed galleys, seventy or eighty were _fustae_, and twenty to twenty-five were _parandaria_. Amid shouts from one ship to another, the beating of drums, and the sound of fifes, all marking the delight of the Turks that their period of inactivity was at an end, the fleet made its way through the Marmora. The sight carried dismay to the remnant of the inhabitants of the Christian villages along the shores, for within the memory of none had such a fleet been seen. Within the city itself the news of the enormous number of vessels on their way was not less alarming. The fleet arrived in the Bosporus on April 12 and anchored at the Double Columns or Diplokionion just below the present Palace of Dolma Bagtche.[229]