Ravenna's tallest campanile was added to the church in the late 10th century. The eastern apse has a seven-sided outer polygon pierced by five windows and flanked by projecting chapels with absidioles.

Bishop Ursicinus (reigned 534-536) founded this church at Classis, about 5 km south of Ravenna; it was paid for by a local banker, Julianus Argentarius, and consecrated in 549 by Bishop Maximian. It is a spacious, three-aisled basilica ending in an eastern apse. Ravenna's tallest campanile was added to the church in the late 10th century. Despite extensive repairs to the clerestory in the 8th century, and the insertion of a crypt and raising of the apse level above the nave floor in the 9th century, the church has largely preserved its original internal design. Much of the sculptural and wall-mosaic decoration has also survived. One of eight Ravenna sites on the UNESCO World Heritage List.