Fort l ' Écluse The Fort l ' Écluse ( or Fort de l ' Écluse ) is close to the village of Collonges , Ain in Eastern France . It commands the Rhone valley and is a natural entrance into France from Switzerland between the Vuache hills and the Jura Mountains . The fort was founded by a Duke of Savoy , and completed by Vauban in the reign of Louis XIV . It was destroyed by the Austrians in 1815 , but was rebuilt by the French , and at the same time considerably strengthened and heightened . The high - road intersects a portion of this fortification . Description and construction . The lower fort ( " fort inférieure " ) , near the level of the Rhône , was rebuilt between 1816 and 1828 . The " route national " 84 passed through the fort until 1939 when it was relocated to a tunnel through the rock behind the fort , defended by a casemate in the tunnel built as part of the Maginot Line fortification program . The casemate was intended to receive 47mm anti - tank guns , but was never equipped . It is linked to the lower fort by a tunnel . The Fort l ' Écluse was the largest component of the otherwise weakly fortified Defensive Sector of the Rhône . The upper fort ( " fort supérieure " ) is above the lower fort . Its location allows the upper fort to control the valley , including the railroad viaduct , and to protect the lower fort . It was built between 1934 and 1848 . Today , the lower fort and the subterranean steps and casemates cut into the rock above it , up to and including terraces just below the upper fort , are open during the summer months for visiting , however the upper fort is not currently safe for visitors . Furthermore , a moderately difficult ( AD ) via ferrata exists which takes climbers from the lower fort to the upper fort by traversing the rocky hillside . In addition , there are many hiking trails in the area which allow hikers access to the upper fort . History . The first fortifications at the site were built by the Romans around 58 BC against incursions by the Helvetii . The fortifications were not sufficient , leading to direct war between the Romans and Helvetii . The defile became a significant trade route in following years . In 1184 a church and residence belonging to the Saint - Claude abbey was built on the site . During the Middle Ages tolls were collected for passage . In the 13th century a fortified house was built at the site by the Sire de Gex . In 1293 the house was ceded to Amadeus V , Count of Savoy . In 1601 the Treaty of Lyon placed Gex in the hands of the French crown . The fort was greatly expanded in the 17th century . The fort played a part in preventing French Protestants from leaving France following the revocation of the Edict of Nantes in 1685 . The fort was burned in 1815 by Austrian forces and reconstructed in following years . The upper fort was built , linked to the lower fort by a tunneled stairway with 1165 steps . By 1860 Savoy became a permanent part of France and the fort was no longer of importance in controlling the frontier . During World War I the fort served as a military training center . In the 1930s the fort was incorporated into French frontier fortifications as a contingency against a German incursion through Switzerland . However , the Germans ultimately arrived from the west , rather than the east . The fort held the passage of the Rhône against German forces in June 1940 during the Battle of France . A single company of the 179th Alpine Fortress Battalion , augmented with an artillery battery and personnel from a pioneers regiment under Lieutenant Mestrallet held the fort with 250 men against Panzergruppe von Kleist , composed of the 3rd and 4th Panzer Divisions and the 13th Motorized Infantry Division . For three days from 22 June the French force held the German advance . After the 25 June armistice , the fort ' s garrison stood its ground until directly ordered by the French command to stand down and to surrender . Present status . The Fort l ' Écluse was purchased by a syndicate of nineteen communities of the " Pays de Gex " and is operated as a museum . It has been open to the public since 1995 .