Der Westwall [The Siegfried Line] (1939)

Known in Germany as the Westwall but referred to by the Allies as the "Siegfried Line", was built during the 1930s opposite the French Maginot Line.
This line stretched more than 630 km (390 mi) and featured more than 18,000 bunkers, tunnels and tank traps.
The network of defensive structures stretched from Kleve on the border with the Netherlands, along the western border of the old German Empire, to the town of Weil am Rhein on the border to Switzerland.
It was planned in 1936 and built between 1938 and 1940.