The Fort Sluys or Fort of the Lock and formerly Fort de la Cluse is a fortified military structure, the xvi th and xvii th centuries profoundly altered in 1830, built on the mountainside to control the parade of the Lockcrossing the Rhône output western Geneva basin in France . This is a classified site that offers the remains of centuries of military architecture, towers, lookouts turrets, loopholes, watchtowers, bunkers, ammunition chambers, etc. It was built by reusing the remains of the ancient castle of lords of Gex founded between 1225 and 1277, known under the name Castle of the Cluse de Gex .