Fort du Petit Bé was built in the 17th century as part of the defensive belt designed by Vauban to protect the city of Saint-Malo from attacks by English or Dutch squadrons. Intended to cross fire with Fort Harbour to defend the entrance to the Rance estuary, it combined, on the sea side, a large horseshoe battery pierced with nineteen embrasures and a watchtower surmounted by a fleur-de-lys and, on the land side, a high fortification crowned with battlements of musketry and composed of two demi-bastions. Armed with nineteen cannons and two mortars, it could accommodate up to one hundred and sixty soldiers.