Baptistery of St. John, Poitiers

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:

1. Andre Grabar, L'Âge d'or de Justinien, de la mort de Théodose à l'Islam, Paris 1966
2. https://en.wikipedia.org/wiki/Baptist%C3%A8re_Saint-Jean