This fascinating basilica provides a vivid glimpse into Rome's multilayered past: a 12th-century basilica built over a 4th-century church, which stands over a 2nd-century pagan temple and 1st-century Roman house. Beneath everything are foundations dating from the Roman Republic.
The medieval church features a marvellous 12th-century apse mosaic depicting the Trionfo della Croce (Triumph of the Cross) and some wonderful Renaissance frescoes in the Chapel of St Catherine.
Steps lead down to the 4th-century basilica inferiore , mostly destroyed by Norman invaders in 1084, but with some faded 11th-century frescoes illustrating the life of San Clement. Follow the steps down another level and you'll come to a 1st-century Roman house and a dark, 2nd-century temple to Mithras, with an altar showing the god slaying a bull. Beneath it all, you can hear the eerie sound of a subterranean river, running through a Roman Republic–era drain.