The interior is faced with black and white marble and has a gilded ceiling and a frescoed dome. It was largely redecorated after a fire in 1595, which destroyed most of the medieval art works.

The cathedral is a basilica with double aisles of different widths, a matroneo ("women's gallery") and an apsidal east end. The new façade was begun (ca. 1110-1125) by Rainaldo and his workshop, who probably also lengthened the nave by three bays. The cathedral was consecrated in 1118 under Pope Gelasius (reigned 1118-1119). The façade was possibly completed ca. 1165 by Guglielmo and his school.