The Life of Cesare Borgia by Rafael Sabatini

Andrea Doria did not remain to make formal surrender of the citadel of Sinigaglia to the duke--for which purpose, be it borne in mind, had Cesare been invited, indirectly, to come to Sinigaglia. He fled during the night that saw Vitelli and Oliverotto writhing their last in the strangler's hands. And his flight adds colour to the versions of the affair that were afforded the world by Cesare and his father. Andrea Doria, waiting to surrender his trust, had nothing to fear from the duke, no reason to do anything but remain. Andrea Doria, intriguing against the duke's life with the condottieri, finding them seized by the duke, and inferring that all was discovered, had every reason to fly.

The citadel made surrender on that New Year's morning, when Cesare summoned it to do so, whilst the troops of the Orsini and Vitelli lodged in the castles of the territory, being taken unawares, were speedily disposed of. So, there being nothing more left to do in Sinigaglia, Cesare once more marshalled his men and set out for Città di Castello-- the tyranny of the Vitelli, which he found undefended and of which he took possession in the name of the Church. Thence he rushed on towards Perugia, for he had word that Guidobaldo of Urbino, Fabio Orsini, Annibale and Venanzio Varano, and Vitelli's nephew were assembled there under the wing of Gianpaolo Baglioni, who, with a considerable condotta at his back, was making big talk of resisting the Duke of Romagna and Valentinois. In this, Gianpaolo persevered most bravely until he had news that the duke was as near as Gualdo, when precipitately he fled-- leaving his guests to shift for themselves. He had remembered, perhaps, at the last moment how narrow an escape he had had of it at Sinigaglia, and he repaired to Siena to join Pandolfo Petrucci, who had been equally fortunate in that connection.