Constructed in the 4th and 5th centuries on Roman foundations, this ginger-stone baptistery formed part of the episcopal ensemble with the cathedral, 100m south. It was redecorated in the 10th century and used as a parish church. The octagonal hole under the frescoes was used for total-immersion baptisms, practised until the 7th century.
It is said to be the oldest remaining Christian building in France.