Built in the 9C on the ruins of a 4C building, this Romanesque church with its solitary campanile has a splendid mosaic floor from the 4C, which is one of the most lavish examples of such a floor in the whole of western Christianity. The wooden framework and the arcades are from the 14C. The cripta degli Affreschi is home to superb Romanesque frescoes; and a second crypt, cripta degli Scavi contains excavated objects including some 4C mosaic slabs.