It is named in honour of the Siena-born architect commissioned by Federico da Montefeltro in 1479 to redesign the layout of the mediaeval keep. Now a museum, it remains an important example of military architecture. During the late 14th century it was conquered by the Malatesta, who succeeded the Montefeltro. In 1502 the fortress was conquered by Cesare Borgia, called the Valentine, but a year later returned to the Montefeltro and in 1527 to the Della Rovere. In 1631, when it passed under the Papal State, it was turned into a jail where numerous Risorgimento patriots were imprisoned, although its most famous prisoner was Count Cagliostro.