The rich mosaics feature Christ Pantokrator and tiers of royal figures and bishops. The Byzantine mosaic decoration was completed before 1170.

The church, dedicated to the Saviour and SS Peter and Paul, was founded by Roger II in 1129 in fulfilment of a vow. An inscription once set in the wall behind the main altar recorded that the church was completed between 1131 and 1148. The building was consecrated on 10 April 1267. The discrepancies in height between the nave and choir and the ambiguous documentary evidence have led to much controversy over the dating of the various parts of the cathedral. The church has three apses, an arrangement also seen in Sicily at Monreale Cathedral. It has an aisled, basilican nave of seven bays with a pointed arcade resting on columns.