Watch Online Free FlixHQ. In the heart of central Europe are some of the world’s most impenetrable military strongholds. In France, 160 megastructure fortresses still line the country’s borders 3 centuries after they were constructed. As solid as ever, how did they withstand attack after attack? Was the secret in their materials? Their shape? In fact, the strength and resilience of these megastructures is due to the genius of one man: Sébastien le Prestre de Vauban.