Since its foundation, the cathedral has been built and rebuilt many times, incorporating Romanesque, Gothic and Neo-Gothic elements. This architectural evolution can be seen in its façade, which encompasses the styles of the 5th through to the 18th-century.
Around the year 500 AD, under the Bishop Basilius, a group of monastic buildings was constructed on top of the old Roman forum. Legend says that they were more specifically built on top of a Roman temple dedicated to Apollo, suggested by the remains of a Roman wall and the columns of the baptistery.
The original Romanesque front of Saint Mary's nave was destroyed in the 15th-century and replaced with a new Gothic façade and bell tower. The interior naves also display a mixture of styles, with the 6th-century baptistry contrasting with the recently restored cloister, which dates back to the 12th-century. Painted and sculpted neo-Gothic decoration was added to the nave in the 19th-century.