The architectural and artistic history of the cathedral derives from overlapping of countless building works that took place here from Roman times up to the 19th century. Ancient tradition assigned the commissioning of the Romanesque church to Bishop Anselmo, who held the See from 994 to 1026. The archaeological excavations carried out from 1976 up to the present day and tree-ring dating analyses have confirmed this tradition, indicating that the construction works began in the late 10th century and lasted until the second half of the 11th. The edifice was of basilican plan with a central nave and two side aisles without a transept, and the entrance was on the south side. The east side ended in an imposing structure articulated on various levels with five apses; the choir, raised over a crypt with three aisles, terminated in a large semi-circular apse, flanked by the apses of the side aisles and those of the two bell towers.