The cultural influences of Sicily collide in this palace dating back to the 8th century b.c., when Punic administrators set up an outpost in the highest part of the city. In the 9th century a.d. the Arabs built a stronghold on the spot for their emirs and their harems, and in the 12th century the Normans turned what was essentially a fortress into a sumptuous royal residence. Here Frederick II presided over the early 13th-century court of minstrels and literati that founded the Scuola Poetica Siciliana, marking the birth of Italian literature. Spanish viceroys took up residence in 1555, and today most of the vast maze of rooms and grand halls houses the seat of Sicily’s regional government.

Arab–Norman cultural influences intersect most spectacularly in the Cappella Palatina, a chapel covered in glittering Byzantine mosaics from 1130 to 1140. Work was finished in time for the coronation of Roger II, who proved to be not only the most powerful of European kings but also the most enlightened. High in the cupola at the end of the apse is Christ Pantocrator (holding the New Testament in his left hand and making the blessing with his right hand), surrounded by biblical characters, some interpreted a little less piously than usual—an unremorseful Adam and Eve happily munch on the forbidden fruit and greedily reach for a second piece. Shame prevails in the next scene, when God steps in reproachfully and the naked couple cover themselves. The mosaics are vibrant in the soft light, an effect especially powerful in scenes depicting water—in the flood and the Baptism of Christ, the water actually appears to be shimmering.

Scenes on the wooden ceiling were done in a 3-D technique using small sections of carved wood, known in Arabic as muqarnas. A team of Egyptian carpenters and painters created the playfully secular scenarios of dancers, musicians, hunters, drinkers, and banqueters in a harem. They're best seen with binoculars or a telephoto lens.

The Royal Apartments are open to the public when the Sicilian parliament is not in session. Tuesdays through Thursdays, legislators meet in the Salone d’Ercole, named for its mammoth 19th-century frescoes depicting the twelve labours of Hercules (perhaps an apt emblem for legislators wading through government bureaucracy). Rooms from the years of Spanish rule are fairly pompous, but earlier eras are also represented, as in the Sala dei Presidenti, a stark medieval chamber hidden in the bowels of the palace for centuries, completely unknown until it was exposed by a 2002 earthquake. The Torre Gioaria (Tower of the Wind) provided a 12th-century version of air-conditioning: A fountain in the middle of the tower (since removed) spouted water that cooled the breezes coming from the four hallways. Much less hospitable are the Segrete, or dungeons, where the cold stone walls are etched with primitive scenes of Norman warships. The otherwise enlightened Frederick II allegedly took his interest in science to perverse lengths in these chambers, shutting prisoners in casks to see if their souls could be observed escaping through a small hole at the moment of death. Frederick was also fascinated by the stars and brought many astronomers and astrologers to his court. His Bourbon successors shared the interest and in 1790 added an astronomical observatory, still functioning, at the top of the Torre Pisana. From these heights in 1801 the priest Fra Giuseppe Piazza discovered Ceres, the first asteroid known to mankind.