The other incident, growing out of the naval defeat, was that Mahomet, on finding that his small craft, propelled only by oars, were of little effect against the powerful vessels at the disposal of the Greeks, determined to transfer a large number of them from the Bosphorus to the upper part of the harbour, where the bigger vessels could not engage them, owing to the shallow depth of water, and where they would be of use against the inner defence of the city. For this purpose Mahomet directed the construction of a broad plank road from Tophane, on the Bosphorus, across the hill intervening between it and the head of the Golden Horn. This road was well greased with tallow, and the vessels were dragged up it with windlasses and oxen. The descent on the other side of the hill was easy enough. The scheme was not quite a novelty, as an operation of the same kind, though on a smaller scale, had been attempted elsewhere. It was carried out with striking success; and in one night eighty of the Turkish galleys were transferred in this way to the upper harbour. Mahomet also constructed a pontoon bridge across the harbour, on which batteries were erected. The two schemes together enabled him to attack the Greek defences along the line of the harbour, and compelled Constantine to withdraw many men from the defence of the landward walls, where the main attack was made. The young Sultan took a most active part in the siege work. He traced the lines of fourteen batteries from which the walls were bombarded. The first great cannon was a failure. It burst at the first shot and blew to pieces the Wallachian who had cast it. It was recast, however, and two others of the same size were also cast. About two hundred smaller guns were used. They threw stone balls[18] against the walls and towers of the city, and ultimately succeeded in effecting a breach. There can be no doubt that the capture of the city was mainly due to the provision of these great guns, which were far above anything previously used against fortresses. The Greeks also used cannons in defence, but the parapets of the walls were not wide enough to allow of the recoil of the guns, and where it was possible to use them the walls suffered from the concussion. Gunpowder was also deficient. After seven weeks of siege the bombardment effected breaches in the walls at three points such as to give Mahomet every hope of success in a final assault. The principal breach was at St. Romanus, where the outer of the two main walls was practically levelled for a length of four hundred yards, and four of the flanking towers were destroyed. The broad ditch was filled in part by the débris of the wall and in part by fascines. The Sultan decided that the assault should take place on May 29th. This became known to the Greeks in the city, and both sides made every preparation for a supreme effort.