This isn’t just another Roman church—far from it. In this layered church-upon-a-church, centuries of history peel away. In the a.d. 4th century, a church was built over a secular house from the 1st century, beside which stood a pagan temple dedicated to Mithras (god of the sun). Down in the eerie grottoes (which you explore on your own), you’ll discover well-preserved frescoes from the 9th to the 11th centuries. The Normans destroyed this lower church, and a new one was built in the 12th century. Its chief attraction is the mosaic adorning the apse, as well as a chapel honoring St. Catherine of Alexandria with 1428 frescoes by Masolino.