As treaties were fruitless, the siege of Città di Castello began. Niccolò defended himself valiantly, and inflicted great injuries on his opponents, who, about six thousand men strong, far exceeded his forces. It was said that Florence and Milan had supported him in his opposition. It was certainly the interest of Florence not to let a State of doubtful proclivities rise up so immediately on her frontiers. In the middle of July the determination was taken to enlist 3,000 horse and as many foot for the protection of the territory, and to send Piero Nasi as commissary to the neighbouring Borgo San Sepolcro. At the first voting the resolution passed with a large majority. The Pope took the demonstration very ill. At last the Vitelli saw themselves obliged to come to an agreement, especially after Federigo of Montefeltro, on whom Sixtus IV. bestowed the title of Duke of Urbino, threw his great military authority into the scale in favour of the Pope, a movement to which King Ferrante assented. The conditions were extremely favourable to the town, but Niccolò Vitelli was obliged to quit it; and though he was honourably received in Rome, and obtained from Feltrier, a no less honourable position in Urbino, his power was at an end. An attempt made in the following year to regain it by force made his situation worse, as he was now obliged to flee, and his rival, Lorenzo Giustini, it is true under a papal governor, assumed the highest rank among the citizens.[203] The Cardinal della Rovera openly accused the Republic of Florence[204] of having assisted the Vitelli against the Pope, increased the vigour of his opposition, and thus led to a capitulation, of which Cardinal Ammanati, notwithstanding his attachment to Florence and the Medici, wrote that it was a disgrace for the victors, for not they, but the vanquished, dictated the terms.