More recent excavations have revealed architectural decorations, remains of a baptistery and a mosaic under the sanctuary, which point to an early Christian predecessor of the 5th or 6th century. Some features that continue to indicate a great age of the church are also evident in the rising masonry of the existing building. These include the polygonal exterior shape of the central apse and (possibly secondary) capitals in the interior.
In the late 8th/early 9th century, the church was extensively rebuilt and basically got its current appearance of a three-aisled basilica six bays long. The side aisles, halfway across the main nave, are separated from each other by arched positions of five pairs of columns. A richly ornamented ciborium above today's main altar has been preserved from the furnishings of that time. Another conversion took place in the 11th century, as a result of which the two side apses were added. In the middle of the 12th century the western part of the church was rebuilt. Pope Alexander III inaugurated in 1177 the facade, which has a strong Tuscan influence, was built in alternating white limestone and red breccia and probably superimposed on the original west end. The upper part of the façade, which was also originally planned in arcade positions, was no longer implemented. According to a building inscription, the construction measures were not completed until 1278. The Romanesque portal, from which the column bases have been preserved, probably had a prothyron, a small vestibule. It was replaced from 1490 as part of a redesign of the facade. The expressive Pietà by the sculptor Petar from Trogir in the round arch above the portal dates from 1514.