Few people do, but it's worth stopping off at this fortified church to check out its lovely cloister and medieval frescoes. These 13th-century paintings, in the Oratorio di San Silvestro, depict the story of the Emperor Constantine, Pope Sylvester I and the so-called Donation of Constantine, a notorious document with which the emperor supposedly ceded control of Rome and the Western Roman Empire to the papacy.
Also worth seeing, though you'll have to sign up for one of the bimonthly guided tours (€10; aulagoticasantiquattrocoronati.it), is the frescoed Aula Gotica (Gothic Hall).
The basilica, which dates from the 6th century, took on its present form in the 1100s after the original was destroyed by the Normans in 1084. Its name – the Basilica of the Four Crowned Martyrs – is a reference to four Christian sculptors who were supposedly killed by Emperor Diocletian for refusing to make a statue of a pagan god.