Work on the Cathedral began at the end of the 12th century. The walls of the north and east wings of the cloister are of Romanesque ashlars on the outside, but inside you can still see the corner of the facade of a Roman building made of large ashlars, which begun in the 2nd quarter of the 1st century AD and completed in the times of Vespasian (69-79).

Over the centuries, various chapels have been opened up in the lateral naves. These demonstrate the chronological and stylistic evolution of both the interior and the exterior of the Cathedral.