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.

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 200 metres (660 ft) 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 1834 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.

In his Commentaries on the Gallic War, Julius Caesar describes the defile as "narrow and difficult ... by which carts could be scarcely drawn one at a time, and moreover a very high mountain overhung it, so that very few could easily defend it." In their attempt to migrate to southwestern Gaul in 58 BC, the Helvetii initially tried to avoid this route by asking Caesar's permission to cross the Rhone into Roman territory at Geneva; when Caesar refused, they attempted to force their way across at several places, only to be rebuffed by a line of fortifications nineteen Roman miles long that Caesar had hastily constructed along the southern bank of the river. In the end, the Helvetii secured safe passage through the defile via an exchange of hostages with the Sequani, who inhabited the area.

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