At the end of the 4th century and beginning of the 5th century a large episcopal complex with the basilica and anexes was erected on the space of the Roman Forum.

The sacristy next to the cathedral has a beautiful mosaic and the baptistery is an unique hexagonal building .The basilica itself is also impressive with its white and green columns, ancient paintings (frescos) and beautiful mosaics. St Stephen's basilica (St. Simeon's church), with a series of biforas on the southern wall has preserved its structure from the 5th century. The impressive rotunda of St. Donat dominates the town and features monolithic columns.

In the early Middle Ages Zadar was one of the main towns of the Byzantine territory of Dalmatia(Thema).