As an architectural complex, it is characterized by the presence of different styles, due to a long history of additions, alterations and restorations, the last of which occurred in the 18th century.

The church was erected in 1185 by Walter Ophamil, the Norman archbishop of Palermo and King William II's minister, on the area of an earlier Byzantine basilica.

Two lintelled ogival arcades, stepping over the street, connect the western façade to the bell tower, which is annexed to the Archbishops Palace (now Museo Diocesano).

The south side has outstretching turrets and a wide portico (the current main entrance) in Gothic-Catalan style, with three arcades, erected around 1465 and opening to the square.

To the right, in the presbytery, is the chapel of Saint Rosalia, patron of Palermo, closed by a richly ornamental bronze gate, with relics and a 17th-century silver urn which is object of particular devotion.

The crypt, accessed from the left side, is an evocative room with cross vault supported by granite columns, housing tombs and sarcophagi of Roman, Byzantine and Norman ages.

People buried here include archbishops Walter Ophamil, the church's founder, and Giovanni Paternò, patron of Antonello Gagini who sculpted the image on his tomb.

The Cathedral Treasury contains goblets, vestments, monstrances, a 14th-century breviary and the famous Crown of Constance of Sicily, a golden tiara found in her tomb in 1491.

The cathedral has a meridian, which may be considered as an earlier type of heliometer (solar "observatory"), one of a number constructed in Italian churches, mainly in the 17th and 18th centuries.

The instrument could be used for research on the apparent movements of the Sun, including its relative distance from the Earth, measured through the size of the floor projected solar disk, and even to compare the results with Ptolemaic and Copernician predictions.

The reworkings by Marvuglia were in reality much more invasive and radical than the projects of the Florentine architect, who thought instead of keeping, at least in part, the complex of longitudinal aisles and the original wooden ceiling.

The restoration intervened to change the original appearance of the complex, providing the church of the characteristic but discordant dome, performed according to the designs of Ferdinando Fuga.

Other precious objects, enamels, embroidery and jewelry, are exposed in central message boards such as for example the breviary parchment of the 1452 coat of arms with an Archbishop Simon from Bologna.