Fort Douaumont

Fort Douaumont was the largest and highest fort on the ring of 19 large defensive works which had protected the city of Verdun, France, since the 1890s. By 1915, the French General Staff had concluded that even the best-protected forts of Verdun could not withstand bombardments from the German 420 mm Gamma guns. These new super-heavy howitzers had easily taken several large Belgian forts out of action in August 1914.

Source: Wikipedia