In military science, a blockhouse is a small, isolated fort in the form of a single building. It is intended to serve as a defensive strongpoint against any enemy which does not possess siege equipment or, in modern times, artillery. A fortification intended to resist these weapons is more likely to qualify as a castle, or in modern times, a bunker.
Blockhouses may be made of masonry where available, but were commonly made from very heavy timbers, sometimes even logs arranged in the manner of a log cabin. They were usually two or even three floors, with all storeys being provided with embrasures or loopholes, and the uppermost storey would be roofed. If the structure was of timber, usually the upper storey would project outward from the lower so the upper storey defenders could fire on enemy attacking the lower storey, or perhaps pour water on any fires. When the structure had only one storey, its loopholes were often placed close to the ceiling, with a bench lining the walls inside for defenders to stand on, so that attackers could not easily reach the loopholes.
Blockhouses were normally entered via a sturdy, barred door at ground level. Most blockhouses were roughly square in plan, but some of the more elaborate ones were hexagonal or octagonal, to provide better all-around fire. In some cases, blockhouses became the basis for complete forts, by building a palisade with the blockhouse at one corner, and possibly a second tower at the opposite corner. Many historical stone blockhouses have survived, and a few timber ones have been restored at historical sites. In New Zealand, a number of one storey timber blockhouses survive from the Maori Wars, while stone blockhouses from the Boer War are relatively common in South Africa.
During the First and Second World Wars many types of blockhouses were built, when time allowed usually constructed of reinforced concrete. The major difference between a modern blockhouse and a bunker is that a bunker is constructed mostly below ground level while a blockhouse is constructed mostly above ground level.
Some blockhouses like those constructed in England in 1940 were built for traditional fortification reasons, often hexagonal in shape and were called pillboxes. 20,000-30,000 were built in Britain during WWII in preparation for a possible German attack.
In London the Admiralty Citadel is one of the most sturdy above ground structures built during World War II. It was constructed in 1940–1941 as a bomb-proof operations centre for the Admiralty, with foundations nine metres deep and a concrete roof six metres thick. It too was intended to serve as a strongpoint in defending against the feared invasion.
In Berlin and other cities during World War II some massive blockhouses were built as air-raid shelters and anti-aircraft artillery platforms. They were called Hochbunker (lit. "high bunkers") and those which functioned as anti-aircraft artillery platforms were also called Flak Towers. Some were over six stories high; several survive to this day because of the high cost of demolition. The Pallasstrasse air-raid shelter Schöneberg has a post-war block of flats built over the shelter. During the Cold War the shelter was in use as a NATO foodstore.