The first religious edifice on the site was built in the 6th century on a Roman propylaea, using part of the existing structure.

Between the 9th and 11th centuries, two basilicas were erected on the ruins of the old church, the first dedicated to Our Lady of the Assumption and the second, the cathedral, to Saint Justus (San Giusto).

In the 14th century, the two basilicas were joined by means of the demolition of one nave of either basilica and the construction of a simple asymmetrical façade, dominated by a delicately worked Gothic rose window, as ornate as the new bell tower, using the Romanesque debris stones found on the site and friezes of arms.

[1] It contains the tombs of: Among the works of historical interest in the cathedral are the apsidal mosaics depicting Our Lady of the Assumption and Christ between Saints Justus and Servolus, laid by master craftsmen from Veneto in the 12th-13th centuries.

Archaeological excavations carried out here in the 1930s laid bare the remains of the Roman forum and civic building constructed on two colonnaded floors with two apses.