A Renaissance fortress built between 1540–1543 for Pope Paul III, designed by Antonio da Sangallo the Younger. To construct it, numerous Etruscan, Roman, and medieval buildings were demolished, including the homes of the Balioni family in the Santa Giuliana district, as well as over a hundred towers, gates, churches, and monasteries. The previous streets of the historic city center were transformed into underground passages, which are now open to the public, allowing visitors to enter the city on foot.

The remains of the fortress, which were destroyed by the citizens of Perugia after the unification of Italy, remain a symbol of resistance against oppression. Visitors can walk among the ruins, sometimes used for exhibitions, and see some of the archaeological remains.