The lower register of the apse is covered with marble and porphyry inlay (opus sectile), engaged pilasters and a circular seating ledge. At the center is a throne or bishop's seat. The altar is carved with lambs facing the Cross.

The church was begun by Bishop Ecclesius (reigned 521-532) and financed by Julianus Argentarius, who spent 26,000 solidi on its construction; work progressed under bishops Ursicinus (reigned 534-536) and Victor (reigned 538-545), and the church was consecrated in 547 by Bishop Maximian. The basic design is Byzantine in conception and comprises a centrally planned octagon with seven arched niches and a square apsed chancel to the east, resting on eight piers, an enveloping ambulatory and gallery, and a narthex set obliquely to the church. The interior is richly decorated in marble and mosaic. It is the only major church from the period of the Emperor Justinian I to survive virtually intact to the present day and is one of eight Ravenna sites on the UNESCO World Heritage List.