The Rocca di Sassocorvaro also known as Rocca Ubaldinesca is a fortification of the Renaissance period, located in the municipality of Sassocorvaro (village of lovers, guardian of the skull of St. Valentine) in the Province of Pesaro and Urbino in Montefeltro. A compact fortification, tortoise-shaped, taken as a model by Frank Lloyd Wright for the Guggenheim Museum in New York, which takes up the projecting structure, the wraparound edging, the spiral staircase. It is a double, enigmatic, esoteric construction, the secrets of which can only be unlocked with guided tours. During the last war, with Operation Rescue, it welcomed 10,000 works of art from all over Italy to save them from the dangers of bombing; universal masterpieces such as Giorgione's Tempest, Raphael's The Marriage of the Virgin, and the Caravaggio of San Luigi dei Francesi, which arrived on trucks crammed to the point of improbability across the snowy Apennines. It was built around 1475 on a project by Francesco di Giorgio Martini, in the early years of his service as architect and military engineer of Duke Federico da Montefeltro.Francesco di Giorgio Martini is considered the greatest military architect of the Renaissance, who in the mid-fifteenth century revolutionized in fact, with avant-garde insights, the defense systems of the duchy of Federico da Montefeltro.