Fortification evolved as man tried to defend his territory from attack. From fenced-in hills to walled cities to impenetrable forts, strongholds of the past echo the history of battles for territorial control. In a 2-hour special, we examine various historical defenses, including France’s Maginot Line, a defensive string of forts with enfilading firepower; the Atlantic Wall, 3,000 miles of shore fortifications built by Germany in WWII; and the North American Aerospace Defense Command (NORAD).