Francesco di Giorgio Martini, the greatest military architect of the Renaissance, who in the mid-fifteenth century revolutionized the defense systems of the duchy of Federico da Montefeltro with avant-garde intuition. His most beautiful work? Probably the fortress of Mondavio, emblem of the perfect fortress, with its mighty octagonal donjon with irregular faces, the fleeting elevations, the sharp edges to elude the impact of bullets; a complex war machine, designed to resist any type of attack launched by both jet and firearms.