The Venetian ships in the harbour determined to attack the Turks before they could complete the great pontoon which they were preparing to bring up. For some days, however, nothing was done. The attacks on the land walls continued and were beaten back, often with heavy loss. But each day provisions were growing less and the defenders were growing weaker. On the morning of April 28th two Venetian galleys, three smaller ships, and two stored with fire, advanced upon the Turks. They were received with the fire of four cannon. The great galley of Gabrielo Trevisani sank, and one of the smaller ships. Only one of the Turkish ships caught fire. The Venetians who swam to shore when their ships sank were beheaded next day in sight of the defenders of the walls. A bitter revenge was taken. Over two hundred and fifty Turks had at some time or other been captured and lay in the prisons of Constantinople. They were now all beheaded on the walls in sight of their kindred. The horrible act made certain what would be the fate of the city if it fell.
And internal dissensions made the fall seem imminent. The Venetians accused the Genoese and the Genoese the Venetians for the failure of their attack on the Turkish fleet, till Constantine himself called their leaders before him and besought them to be at peace. "The war without," he said, "is enough; by the mercy of God seek not war among ourselves." "So," says Phrantzes, "with much speech at length he pacified them."
Next day and on the first of May the Turkish cannon did some damage; but in some parts the fire was utterly unable to penetrate or dislodge the splendid masonry, and one tower near the Lycus, it is said, was struck by over seventy balls without suffering in the slightest; and the great gun built by the Hungarian mercenary Ourban was dismounted by the fire of the cannon directed by the gallant Genoese engineer Giustiniani, who, with four hundred of his countrymen, manned the walls near the gate of S. Romanus. Mohammed himself was standing by the gun at the moment, and in rage called his troops at once to the assault. They crossed the counter-scarp and began to pull down the scarp where it had been repaired; but again the defenders drove them back.