The Basilique Saint-Pierre-aux-Nonnains has the strongest historical claim of any church in France: parts of its walls date from the late 4th century, when the building served as a Roman sports gymnasium. In the 7th century the Benedictines converted it into a convent church, and successive medieval modifications gave it the form you see today. The bare, austere interior preserves the layers of its long life — Roman brick, Carolingian stonework, and Gothic detail are all visible.
The building no longer functions as a church and is used as an exhibition and concert space. The acoustics are exceptional and the venue regularly hosts classical music recitals; it is worth checking the programme during your visit. The basilica sits beside the Chapelle des Templiers, a small 12th-century chapel that is one of only a handful of Templar buildings in France.