Begun by Emperor Hadrian in AD 135 as his own mausoleum, Castel Sant'Angelo has variously been a fortress, prison and papal residence. It now plays host to temporary art shows, although the real pleasure of a visit to Castel Sant'Angelo lies in wandering from Hadrian's original spiralling ramp entrance to the upper terraces, with their superb views of the city and beyond.
Be careful, though - Puccini had Tosca hurl herself to her death from here. In between there is much to see: lavish Renaissance salons, decorated with spectacular frescoes and trompe l'oeil; the glorious chapel in the Cortile d'Onore, designed by Michelangelo for Leo X; and, halfway up an easily missed staircase, Clement VII's surprisingly tiny personal bathroom, painted by Giulio Romano. In the summer the passetto - linking the castle to the Vatican - is occasionally open (to the halfway point; you won't find yourself in Benedict's bedroom) and well worth a visit.