In AD 271, Emperor Aurelian took advantage of its position guarding the northern approaches to the city and incorporated it into his new system of walls surrounding the city. As a bastion it protected the city from barbarian attacks, and by the Middle Ages had become a substantial fortress. In times of peril, popes fled here across a secret elevated corridor, the Passetto di Borgo, and stored their most precious riches in the castle’s treasury.

Visitors reach the castle across a pedestrian bridge lined with statues of angels (by Bernini), and ascend to its five floors on a spiral ramp. At its various levels are prison cells, a large collection of weapons, and splendidly decorated papal apartments covered in Renaissance frescoes. At the top is a terrace with stunning views of the city.