Begun in 1386, it took five centuries to complete and rises over the area occupied at one time by the churches of Sta. Tecla and Sta. Maria Maggiore. The Gothic facade of the cathedral was completed under Napoleon I. The most imposing parts of the Duomo are its lateral aspects, its two top crosses, and the apse. In the latter, a powerful impression is made by the three immense Gothic windows of finely carved marble.

The casing, of pink-tinged Italian marble, is to be found on all sides of the structure. At the lower level, it lends character to the small trilobate arches, capitals, and flowers; it also appears on the buttresses and, above them, runs along the crowning row of gigantic statues; above these, it covers the decorated water gutters and, finally, enhances the lacelike ornamental crest.

The exterior of the cathedral is covered with a remarkable profusion of turrets, pinnacles, and more than 3,000 statues. Within are 52 pillars, each over 80 feet (24 metres) tall and more than 10 feet (3 metres) in diameter and bearing, instead of capitals, a crown of statues within their niches.