As WWII began, a series of supposedly impregnable fortresses protected Belgium from invasion. At dawn on May 10, 1940, Germany conducted history’s first glider-borne assault against Fort Eben Emael. Despite powerful guns and heavy armor, the Belgians neglected to defend the roof–where nine flimsy, fabric-covered gliders landed and disgorged 71 paratroopers. Using computer graphics, archive footage, and reconstruction shot in the passageways and tunnels, we examine how the fortress fell so easily.