The Sangallo fort was built between 1501 and 1503 by Antonio da Sangallo at the behest of Cesare Borgia, known as Valentino, the son of Pope Alexander VI. In some parts it seems there is the hand of Bramante. The fortress defended Nettuno, at the time considered the "breadbasket of the Lazio", from sea attacks. It was placed in a well-articulated defense system, which linked the coastal towers, the fortress of Ostia in Tor San Lorenzo, Torre d'Anzio up to Torre Astura and so on. The fortress is square, height up to 25 meters and the walls have a thickness of 5 meters with four bastions. Inside the fort is an imposing tower that can accommodate a garrison. July 20, 1925, here Benito Mussolini signed the Convention on the Italians in Dalmatia with Yugoslavia. Today the building is owned by the city and houses the "Museum of the Allied Landing", the Antiquarian, conferences and art exhibitions.