For hundreds of years this fortress guarded the Vatican, to which it is linked by the Passetto, an arcaded passageway. According to legend, Castel Sant'Angelo got its name during the plague of 590, when Pope Gregory the Great (circa 540-604), passing by in a religious procession, had a vision of an angel sheathing its sword atop the stone ramparts. Though it may look like a stronghold, Castel Sant'Angelo was in fact built as a tomb for the emperor Hadrian (76-138) in AD 135. By the 6th century it had been transformed into a fortress, and it remained a refuge for the popes for almost 1,000 years. It has dungeons, battlements, cannon and cannonballs, and a collection of antique weaponry and armor. The lower levels formed the base of Hadrian's mausoleum; ancient ramps and narrow staircases climb through the castle's core to courtyards and frescoed halls, where temporary exhibits are held. Off the loggia is a café.

The upper terrace, with the massive angel statue commemorating Gregory's vision, evokes memories of Tosca, Puccini's poignant heroine in the opera of the same name, who threw herself off these ramparts with the cry, "Scarpia, avanti a Dio!" ("Scarpia, we meet before God"). On summer evenings a book fair with musical events and food stalls surrounds the castle. One of Rome's most beautiful pedestrian bridges, Ponte Sant'Angelo spans the Tiber in front of the fortress and is studded with graceful angels designed by Bernini. COST: EUR5.