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. A fortification intended to resist these weapons is more likely to qualify as a fortress or a redoubt, or in modern times, be an underground bunker.
Read more about Blockhouse: Early Blockhouses in England, Age of Exploration, Second Boer War, Concrete Blockhouses, Afghanistan 2006, Non-military