The cathedral, dating from 1131, was commenced in the Norman style, the island of Sicily having been conquered by the Normans in 1091. According to tradition, the building was erected after a vow made to the Holy Saviour by the King of Sicily, Roger II, after he escaped from a storm to land on the city's beach.
It was probably planned that the entire church should be decorated in mosaic, but it was only completed in the presbyterium area. It still covers the apse and some half of the side walls. Roger II brought masters in the technique of mosaic from Constantinople. They adapted their traditional Byzantine decorative art to an architectural structure that was Northern European origin.
The dominant figure of the decorative scheme is the bust of the Christ Pantokrator, portrayed with a hand raised in Benediction on the semi-dome of the apse. In his left hand he carries the Gospel of John, in which can be read, in Greek and Latin: "I am the light of the world, who follows me will not wander in the darkness but will have the light of life" (John, 8:12). (According to wikipedia)