Over the years, this bulky cylindrical fortress on the Vatican side of the Tiber has had many lives: As the mausoleum tomb of Emperor Hadrian in A.D. 138; as a papal residence in the 14th century; as a castle, where in 1527 Pope Clement VII hid from the looting troops of Charles V; and as a military prison from the 17th century on. Consider renting an audio guide at the entrance to fully appreciate its various manifestations.
From the entrance a stone ramp (rampa elicoidale) winds to the upper terraces, where you can see amazing views of the city and enjoy a coffee at the outdoor cafe. The sixth floor features the Terrazza dell’Angelo, crowned by a florid 18th-century statue of the Archangel Michael. It’s famous to opera fans—the last act of Puccini’s “Tosca” is set here.
From here you can walk back down through five floors. On levels 3 to 5 you’ll see the Renaissance apartments used by some of Rome’s most infamous Popes, including Alexander VI, the Borgia pope. The art collection displayed throughout is fairly mediocre by Rome standards, although there are a few works by Carlo Crivelli and Luca Signorelli, notably a “Madonna and Child with Saints” from Signorelli. Below the apartments are the grisly dungeons (“Le Prigioni”) used as torture chambers in the medieval period (Cesare Borgia made great use of them). The castle is connected to St. Peter’s Basilica by Il Passetto di Borgo, a walled passage built in 1277 by Pope Nicholas III, used by popes who needed to make a quick escape to the fortress in times of danger.
Note that the dungeons, Il Passetto, and the apartments of Clement VII are usually open by guided tour only (English tours Tues–Sun at 10am and 4pm, 5€), with occasional late-night openings in the summer. Classical music and jazz concerts are also held in and around the castle in summertime.