Fort d'Illange, Military fortification in Illange, France.

Fort d'Illange is a fortified structure built on a hillside within wooded grounds. The complex integrates multiple buildings, underground chambers, and passages into a unified defensive system.

Built by Germany between 1905 and 1911 as part of the Moselstellung, eleven fortresses guarding Thionville and Metz against a French attack to retake Alsace-Lorraine. In the 1930s it became the command post of the Fortified Sector of Thionville on the Maginot Line. American forces of the 95th Infantry Division captured it on 12 November 1944 after a night bombardment.

The fort's German name was Feste Illingen, and from 1905 it housed the headquarters of the Thionville fortification group under Generalleutnant von Lochow. After Lorraine returned to France in 1918, the three Thionville forts were regrouped as the Fortified Group of Thionville. Since 2003 the grounds have been run as an open-air museum.

The fort stands on the Grosser Berg hill about 4 km south of Thionville, and the grounds have been run as an open-air museum since 2003. The buildings and underground works are sealed, so visits stay on the surface. The site is abandoned and stripped, with gravel and concrete surfaces and no facilities.

The fort's four 100 mm guns sat in retractable Schumann turrets, controlled from two armoured observation cupolas on the north and south barracks. The east and west barracks also carried cupolas, so the whole hilltop could be watched and swept by fire without exposing gunners.