Built between 1063 and 1170, the Duomo Santa Maria is a melting pot of architectural styles. The cathedral's several architects borrowed its capacious dome and its exterior, black-and-white striped marble motif from 11th-century Moorish architecture. The mosaics inside are Byzantine, while the famous panels on the transept door facing the Leaning Tower are Romanesque. And builders recycled the cathedral's marble from classical buildings, inviting comparisons to ancient Rome.

Its plan is that of a Latin cross, with a 312-foot, doubled aisled nave, transepts, and an apsidal choir. Twenty-four monoliths of red granite support the roof, while the interior holds some 450 columns, some of them dating back to Roman times. A fire in 1595 destroyed much of the cathedral and its original artwork, though its exquisitely carved 14th-century pulpit designed by Giovanni Pisano survived. Roman inscriptions abound, but some of them are upside down, suggesting that their medieval masons could not read what they wrote.