The cathedral was designed and completed between 1215 and 1263 on the site of an earlier structure. It has the form of a Latin cross with a slightly projecting transept, a dome and a bell tower. The dome rises from a hexagonal base with supporting columns. The dome was completed in 1264. The lantern atop the dome was added by Gian Lorenzo Bernini. The bell tower has six bells, where the oldest one was cast in 1149. The nave is separated from the two aisles by semicircular arches. The exterior and interior are constructed of white and greenish-black marble in alternating stripes, with addition of red marble on the façade.