San Clemente is one of Rome's most rewarding layered churches. The medieval basilica stands above an excavated earlier church, which in turn sits above a Roman Mithraeum linked to the mystery cult of Mithras.
The result is a rare vertical walk through Rome's religious history, from ancient underground rooms to medieval mosaics and a working church above. It is close enough to the Colosseum to combine easily, but deserves its own time.