A cylindrical fortress on the Tiber built as a mausoleum for Emperor Hadrian in AD 139, converted in the Middle Ages into a papal stronghold and connected to the Vatican by an elevated passageway (the Passetto di Borgo) used by popes fleeing in emergencies. Now a museum with rooms ranging from Roman-era foundations to lavishly frescoed Renaissance papal apartments at the top.
The roof terrace offers one of the finest views in Rome, looking back along the Tiber toward St. Peter's. The approach bridge, Ponte Sant'Angelo, is lined with Bernini's marble angels and worth crossing in both directions.