Gallo-Roman enclosure of Tours
The Gallo-Roman enclosure of Tours is a wall surrounding the city of Civitas Turonorum (the cathedral quarter of the current city of Tours) and was constructed during the Late Roman Empire. It is commonly referred to as the "castrum enclosure." Only the remaining Gallo-Roman construction in Tours is accessible to the public.
Source: Wikipedia — Gallo-Roman enclosure of Tours (CC BY-SA 4.0)