(Il Duomo). This is a very attractive building. There is considerable Cosmatesque work (geometrical floor mosaics) both inside and out. Uniquely, this duomo has a Cosmatesque facade (signed and dated in gold mosaic tiles, dated 1210) as well as Cosmatesque floor mosaic work. These patterns were created in the Middle Ages from thin slices cut from colored marble and columns left in the ancient ruins. The name derives from a Roman family, the Cosmati, who did this sort of work. Similar Cosmati floors can be found in ], [[Anagni]] and [[Spoleto]]. The high altar is fashioned from a sarcophagus of the 3rd or 4th century. The ancient crypt is fascinating and well worth visiting. Sadly the interior was redone in the 1740s but retains many of its antique marble columns. The organ is the same one on which the young Wolfgang Amadeus Mozart played. Outside, the effect of the beautiful portico, dating back to 1210, is somewhat spoiled by the modern fence that protects it. To see what the original interior would have looked like, visit the Basilica at nearby Castel Sant'Elia.