Located in Corso Vittorio Emanuele, the Cathedral stands on the site of a former Mosque and is dedicated to Our Lady of the Assumption. The exterior is a stunning mix of different architectural styles including Norman, Baroque and Gothic which have been gradually added since the original construction in 1179-85. A lot of the features, including the cupola crowning the roof have only existed since the 1700s.
The interior has also seen many alterations and, like the exterior, reflects many architectural styles. The Chapels are beautiful, particularly the Chapel dedicated to Santa Rosalie.
The Cathedral certainly gives Monreale Cathedral a run for it's money. Admission is free although a donation is appreciated. Closed at lunchtime, the Cathedral is easily walkable from most of the Palermo sights and is one of the main stops on the sightseeing buses. I would definately recommend a visit.