The interior of the cathedral features an ever-increasing decorative rhythm on the surface of the walls and pillars, starting from the main entrance to the altar.
In the space between the naves , the wall area underneath the mosaics is covered in white marble and decorated with geometric polychrome inlays, inserted in vertical bands at regular intervals and enclosed by thin marble cornices. It is not certain whether the aisles were ornamented when the Cathedral was founded, although decoration was certainly part of the design.
The one we see today is the product of recent restorations .
Similarly, the wooden ceiling and the floor of the nave were also unfinished, although they were also completed at a later date in compliance with the client’s design.
The balanced arrangement of the columns that follow one another in the centre of the nave determines the dimensions of this main space, illuminated by nine windows arranged along the same axis as the colonnaded arches but at a lower level than the eastern body, thus influencing its decorative composition. Originally, the light sources , so sensitive to the changing seasons that they could affect the perception of colours inside the Temple, produced a totally different effect from the one we see today. Perforated lead plates on the windows made the interior quite dark for many centuries, until 1658. The need for a new liturgical arrangement made it necessary to replace the lead barriers, obstructing access to the sunlight, with new stained glass windows.
During the restoration carried out between 1957 and 1995, additional metal alloy barriers were added to mitigate the excessive light. As in all mediaeval churches, the sacred space was lit by the glow of candles. In Monreale, the candles glowed softly in the large metal chandeliers supported from above by iron chains.