The original building was erected between the end of the fifth and the beginning of the sixth century, but the oldest elements which can still be seen today date to the period of its reconstruction at the end of the eleventh century. This, Romanesque, phase can be dated to 1118, the year in which Pope Gelasius II consecrated the new church, and is clearly visible in the regularity of the external walls of the main apse and its two sides, in each of which there is a portal.
The lower part of the façade, which remained unfinished, dates to an early thirteenth century phase: the intention was to reconstruct the building to adapt it to the construction methods of the Gothic style of northern France. The work is innovative compared to the French models in the choice to cover the facade using, not only black and white layers, but rather a range of prestigious polychrome marbles: white, red, pink and green, and with stone in the signaling. While in the facing of the fronts and sides of both bell towers, Roman sarcophagi can be seen they date from different periods, types and origins and add character and ennoble the building.
The upper part of the façade has parts dating from various periods: the level of the hexaphores to the fourteenth-century, the fifteenth-century produced the rose window (which was rebuilt in the mid-nineteenth century) and the (unfinished) loggia which crowns the tower; while the bell tower on the right dates to the sixteenth-century being completed in 1522.
Inside, a sort of atrium (endonarthex), completely vaulted on pillars, suggests how the interior of the cathedral would have been had it been renovated according to the thirteenth-century design, and at the start of the two aisles the attachment points for the (unfinished) arcades can still be seen .
Inside, various altars and chapels stand out: in particular along the left aisle the grandiose chapel dedicated to St John the Baptist, with a marble facade dating from the mid-fifteenth century and its interior from the early sixteenth; the three apse chapels whose decoration date respectively from the mid-sixteenth century (left), between the sixteenth and first half of the seventeenth century (main chapel), and the second half of the seventeenth century (right).