With its chunky round keep, this castle is an instantly recognisable landmark. Built as a mausoleum for the emperor Hadrian, it was converted into a papal fortress in the 6th century and named after an angelic vision that Pope Gregory the Great had in 590. Thanks to a secret 13th-century passageway to the Vatican palaces, the Passetta di Borgo, it provided sanctuary to many popes in times of danger, including Clemente VI, who holed up here during the 1527 sack of Rome.

Its upper floors are filled with lavishly decorated Renaissance interiors, including, on the 4th floor, the beautifully frescoed Sala Paolina. Two stories further up, the terrace, immortalised by Puccini in his opera Tosca, offers great views over Rome.