 Complete the following phrase:  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 19311935, it was reputed to be impregnable and at the time, the largest in the world. The fort was 