In military science, a blockhouse is a small, isolated fort in the form of a single building. It serves as a defensive strong point against any enemy that does not possess siege equipment or, in modern times, artillery.
Reichsbahnbunker (aka The Bunker) Berlin-Mitte, Germany - 1943 Karl Bonatz
The bunker was built to house 3,000 Reichsbahn train passengers. The square building has an area of 1000 m² and is 18 metres high. Its walls are up to two metres thick and there are 120 rooms spread across five floors.