Built in the 14C on the site of the palaeo-Christian basilica Santa Restituta (4C), the Duomo was altered many times. The fabulous Cappella del Tesoro di San Gennaro is a showcase of the ornamental magnificence of the Baroque; the chapel houses the treasure of St Januarius, which includes two phials of the saint's blood. A staircase leads down to the superb Succorpo (crypt). The San Giovanni baptistery (4C and 5C) has retained its fine structure and its precious original mosaics.