Fort Eben-Emael

Fort Eben-Emael is an inactive Belgian fortress located between Liège and Maastricht, on the Belgian-Dutch border, near the Albert Canal, outside the village of Ében-Émael. It was designed to defend Belgium from a German attack across the narrow belt of Dutch territory in the region. Constructed in 1931–1935, it was reputed to be impregnable and at the time, the largest in the world. The fort was neutralized by glider-borne German troops on 10-11 May 1940 during the Second World War. The action cleared the way for German ground forces to enter Belgium, unhindered by fire from Eben-Emael. Still the property of the Belgian Army, the fort has been preserved and may be visited.

Source: Wikipedia