It is here that the Emperor Constantine built his first basilica and in the crypt you can still find traces. The cathedral was rebuilt for the first time by Pope Leo III (795-815) after a devastating fire. In 1772 the facade was rebuilt and it has undergone further changes in 1854. The interior of the cathedral is in the neoclassical style, with three naves and six side chapels. It houses a magnificent marble sarcophagus of the early Christian era and numerous paintings of the XVII and XVIII centuries.