On the site of an old Roman "fort", this sanctuary was one of the most vibrant focal points of Christianisation in Savoie in the late Middle Ages. Formerly believed to be a Carolingian baptistry, the small rotunda, with its simple capitalled columns, is now thought to be an 11C reliquary. The chancel of the crypt erected in the 15C to support the upper Gothic church houses an Entombment from the same era, damaged during the Revolution.