6 The fortress of San Leo is in the Marche region and located a few miles west of the Republic of San Marino. It dates to Roman times. First extended and retructured in the Middle Ages, is was redesigned during the Renaissance. It is an oustandind example of military architecture of that time. Its current appearence reflects the strong imprint desired by Federico da Montefeltro who had the fortress changed substantially around the second half of the 1470s. The two vast circular keeps which frame its westward flanks were designed and realized by Francesco di Giorgio Martini, one of Italy's greatest military architects, who was much used by Duke Federico da Montefeltro. During the war with Cesare Borgia (1502), Guidobaldo da Montefeltro had ordered the demolitions of his fortresses so that they could not be usead by the enemy. Fortunately, San Leo was one of the fortresses that escaped the order of demolition.