This bulky cylindrical fortress on the Vatican side of the Tiber has a storied, complex history, beginning life as the tomb of Emperor Hadrian in a.d. 138, and later serving as a castle (Pope Clement VII escaped the looting troops of Charles V here in 1527), papal residence in the 14th century, and military prison from the 17th century (Puccini used the prison as the setting for the third act of “Tosca”). Consider renting an audioguide at the entrance to help fully appreciate its various manifestations. The ashes and urns of Hadrian and his family have long since been looted and destroyed, and most of what you see today relates to the conversion of the structure into a fortress and residence by the popes from the 14th century.

From the entrance a stone ramp (rampa elicoidale) winds its way to the upper terraces, from which 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 statue of the Archangel Michael cast in 1752 by the Flemish artist Peter Anton van Verschaffelt (location of the tragic denouement in “Tosca”).

From here you can walk back down through five floors, including the Renaissance apartments (levels 3–5) used by some of Rome’s most infamous popes: Alexander VI (the Borgia pope) hid away in the castle after the murder of his son Giovanni in 1497, overwhelmed with grief (although his vows of moral reform were short lived).

Below the apartments are the grisly dungeons (“Le Prigioni”) used as torture chambers in the medieval period, and utilized especially enthusiastically by Cesare Borgia. The castle is connected to St. Peter’s Basilica by Il Passetto di Borgo, a walled 800m (2,635-ft.) passage erected in 1277 by Pope Nicholas III, used by popes who needed to make a quick escape to the fortress in times of danger, which was fairly often. Note that the dungeons, Il Passetto, and the apartments of Clement VII are only usually open on summer evenings (July–Aug Tues–Sun 8:30pm–1am; free 50-min. tours with admission, English tour at 10:30pm). Classical music and jazz concerts are also held in and around the the castle and gardens in summer (Wed, Fri–Sun 9:30pm).