[pygy_]

Fellow Belgian here, our ancestors had been naive...

Our main line of defense, the fort d'Ében-Émael, had been built by German workers. The German army had their plans, and knew the weak point of the fort: its vast, undefended roof (it was used as a football field by the soldiers).

During a dark night, they landed with gliders on the roof, and manually set up shaped charges[1] to destroy the turrets, which were resistant to conventional bombs.

Game over.

[1] http://en.wikipedia.org/wiki/Shaped_charge , damages seen from the outside: http://upload.wikimedia.org/wikipedia/commons/thumb/f/f6/Ebe...

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Edited to reconcile my foggy memory with the historic truth...

[gaius]

I'd be willing to bet some young combat engineer said "but what about the roof?" and some old general replied "you'll never get a horse up there!".

[pygy_]

It's worse than that.

The roof should have been mined, but the soldiers petitioned the hierarchy to keep their sports field...