The Bell Tower, which stands to the left of the façade, was probably built between 1173 and 1198, more or less at the time the Cathedral was consecrated, using materials from ancient buildings that had already been reused to build the earlier church on the site.

The Bell Tower incorporates within it another smaller tower also built using materials from the Roman era and the staircase rises up between the two towers. The Bell Tower has been used in a defensive function throughout its history on account of its solidity, height and strategic emplacement, it and the Castle being among the safest structures in the city. The panoramic view of the city and surrounding countryside from the top of the tower is absolutely breathtaking.