Founded by Pope Sixtus III (432-440), this is one of Rome's four great basilicas, and has been remodelled many times down the centuries. Erected in 1377, its campanile is the highest in Rome. The façade is the work of Ferdinando Fuga. The interior is breathtaking; note the beautiful 5C mosaics in the nave depicting Old Testament scenes, and also those decorating the apse and the main arch.