The St. John Baptistery of Poitiers was built ca 350 AD atop the substructures of Roman buildings that were demolished in 276 AD. Inside a rectangular room, surrounded by several annexes, was octagonal pool with three steps.
See:
- Andre Grabar, L'Âge d'or de Justinien, de la mort de Théodose à l'Islam, Paris 1966
- https://en.wikipedia.org/wiki/Baptist%C3%A8re_Saint-Jean
