Swamps as well as wooden palisades provided protection from “barbarian” invasions. From the 11th century, stone came to supplant wood and it was in the XII century that the citadel was built.
The geographical situation naturally lends itself to the establishment of a stronghold. To the east, a steep plateau cut by two small valleys. The Allier, a wild river, forms a natural barrier to invasions from the West. Established on a circular mound 50m above the ground, Billy has a sentinel role to watch over the land of Auvergne, enemy of the Bourbonnais lordship.