Backing onto the Roman Forum, this 6th-century basilica incorporates parts of the Foro di Vespasiano and Tempio di Romolo, visible at the end of the nave. However, the main reason to visit is to admire its fabulous 6th-century apse mosaic depicting Peter and Paul presenting saints Cosma, Damiano, Theodorus and Pope Felix IV to Christ.
Also worth a look is the 18th-century Neapolitan presepe (nativity scene) in a room off the salmon-orange 17th-century cloister.