Stalin Line

Per Wikipedia "The Stalin Line was a line of fortifications along the western border of the Soviet Union. Work began on the system in the 1920s to protect the USSR against attacks from the West. The line was made up of concrete bunkers and gun emplacements, somewhat similar to, but less elaborate than the Maginot Line. It was not a continuous line of defense along the entire border, but rather a network of fortified districts, meant to channel potential invaders along certain corridors."