Fort Eben-Emael
The Blitzkrieg of the Second World War truly showed that fixed fortifications are easily defeated by manoeuvre instead of frontal assault or long sieges. The great Maginot Line was bypassed, and battles that would have taken weeks of siege could now be avoided with the careful application of air power (such as the German paratrooper capture of Fort Eben-Emael, Belgium, early in World War II).