From the original church devoted to S. Flaviano and elevated to the status of Cathedral in 1240, then built over a previous sacred structure, today only very traces remain in the crypt. It was completely rebuilt between 1384 and 1402, thus transforming the ancient Romanesque architecture into a more modern Gothic structure. Worthy of mention are the splendid octagon coffered wooden ceiling that extends for the entire length of the nave, the frescoes and stucco decoration of the apse and the incorporation of the ancient in rectangulat pillars alongside Ionic columns that support low arches.