The archiepiscopal palace and monastic buildings on the south side were of great size and magnificence, and were surrounded by a massive precinct wall, crowned at intervals by twelve towers.

This has been mostly rebuilt, and but little now remains except ruins of some of the towers, a great part of the monks' dormitory and frater, and the splendid cloister, completed about 1200.

On each side, monolithic columns of grey oriental granite (except one, which is of cipolin marble) support eight pointed arches much stilted.

There is no triforium, but a high clerestory with wide two-light windows, with simple tracery like those in the nave-aisles and throughout the church, which give sufficient light.

A large open atrium, which once existed at the west, is now completely destroyed, having been replaced by a Renaissance portico by Giovanni Domenico and Fazio Gagini (1547–1569).

[4] The tomb of William I of Sicily (the founder's father), a porphyry sarcophagus contemporary with the church, under a marble pillared canopy, and the founder William II's tomb, erected in 1575, were both shattered by a fire, which in 1811 broke out in the choir, injuring some of the mosaics and destroying all the fine walnut choir-fittings, the organs and most of the choir roof.

On the north of the choir are the tombs of Margaret of Navarre, wife of William I, and her two sons Roger and Henry, together with an urn containing the viscera of Saint Louis of France, who died in 1270.

The pavement of the triple choir, though much restored, is a specimen of marble and porphyry mosaic in opus alexandrinum, with signs of Arab influence in its main lines.

The mosaic pavement of the nave was completed in the 16th century, and has disks of porphyry and granite with marble bands intermingled with irregular lines.