The fort was built by Pope Alexander VI in the late fifteenth on the spur of Civita Castellana plateau. It was designed by Antonio da Sangallo the Elder and is pentagonal with five bastions, three of which were fitted with gunners, and it is surrounded by an artificial moat on three sides. Pope Julius II della Rovere (1503-1513) instructed the young Antonio da Sangallo to continue his uncle's work by completing the great court, adding the porch to two levels, the octagonal tower and an ashlar portal for access to the Fort. Considered one of the most important works of the military, the Fort was a papal residence until the early 1800s, when it became a prison. During the second world war many families took refuge in the fortress that since 1950 has been a museum.