Eben-Emael was a Belgian fortress between Liège and Maastricht, near the Albert Canal, defending the Belgian-German border. Constructed in 1931–1935, it was reputed to be impregnable and at the time, the largest in the world. But on 10 May 1940, 78 paratroopers of the German 7th Flieger (later 1st Fallschirmjäger Division) landed on the fortress with gliders (type DFS 230), armed with special high explosives to damage the fortress and its guns. As the fortress had no defence against air-attacks or machinegun positions, one day later, when they were reinforced by the German 151st Infantry Regiment, at 13:30 h on 11 May, the fortress surrendered. 1200 Belgian soldiers were captured.