The diocese was established by the late 5th century,[2] but the structure of the present Romanesque cathedral building dates from 1080, when construction was begun under archbishop Arnald of Cluny.

The campanile was added in 1555, and reuses many interesting fragments of ancient stonework, including two Roman sarcophagi and a sacrificial altar decorated with bulls' heads.

The interior of the cathedral features many unusual zoomorphic and floral carvings, in the Cluniac manner, the possible symbolic significance of which has caused speculation.

[3] Of especial note are a polyptych showing Our Lady of the Rosary and the Fifteen Mysteries, with Saints Dominic and Thomas, made by Antonio Stabile in 1583, and four frescos in the cloister by Giovanni Todisco di Abriola.

The cathedral also possesses a noted marble portrait bust of the Emperor Julian the Apostate, which used to be believed to show Saint Canius.