Basilica San Clemente

Named after Pope St. Clement, the basilica is a layer cake of Roman history and architecture—a private home turned Christian church in the second century, which morphed into a Mithras temple in the third century, and its current incarnation as a Medieval temple after that. While it may not look like much on the outside, inside it’s done in breathtaking twelfth-century mosaics with multiple chapels and fascinating little nooks making up a sort of architectural treasure hunt.