Arms and Armour
A blockhouse is a small fortification, usually consisting of one or more rooms with loopholes, allowing its defenders to fire in various directions. It is usually an isolated fort in the form of a single building, serving as a defensive strong point against any enemy that does not possess siege equipment or, in modern times, artillery, air force or cruise missiles. 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. However, a blockhouse may also refer to a room within a larger fortification, usually a battery or redoubt.
Knights Hospitaller.
Battle of Fort Sumter.
Text from Wikipedia; plate via Wikimedia Commons. Text CC BY-SA 4.0; plate freely licensed (see Commons). Source record. Images and catalogue data are reproduced from open-access collections.