The cathedral, dedicated to the Virgin Mary, can be considered one of the finest examples of Romanesque architecture in Italy. It was erected by the heretic bishop Cadalus, who later became antipope Honorius II. Destroyed by an earthquake in 1117, it was rebuilt and completed in the 12th century. The tall bell tower, topped by a gilt copper angel, was added in the following century and the side chapels during the 14th and 15th century.
The facade is made from blocks of sandstone and decorated with a row of loggias and two tiers of galleries. The porch on the main door is supported by lions, built in 1281 by master stone-cutter Giambono da Bissone. The octagonal dome, mounted on a crossing tower is quite unusual for a medieval church. 
The interior of the Cathedral is shaped on a Latin cross. The ceiling and walls are frescoed in the Mannerist style. A 16th century red Verona marble staircase leads up to the transepts where, on the right, is the famous relief of the Deposition by Benedetto Antelami, one of the finest examples of Romanesque sculpture, showing clear Provencal influences. The great void of the dome was frescoed by Correggio in 1526 with the Assumption of the Virgin. Concentric circles of clouds and heavenly hosts that inspired much of the Baroque work of the following century thanks to its illusionistic style so ahead of its time. The vaults above the choir were painted by Girolamo Mazzola Bedoli (1538) who also frescoed the Last Judgment in the apse semidome.