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.
Originally blockhouses were often constructed as part of a large plan, to "block" access to vital points in the scheme. But from the Age of Exploration to the nineteenth century standard patterns of blockhouses were constructed for defence in frontier areas, particularly South Africa, New Zealand, Canada, and the United States.
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 enemies 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 New Zealand land wars.
During the Second Boer War the British forces build a large number of fortifications in South Africa, most of these were cheap corrugated iron structures, but around 441 were solid masonry blockhouses, many of which still stand today. A range of designs were used in the construction of these blockhouses, but most were either two or three story structures built using locally quarried stone.
These blockhouses played a vital role in the protection of the railway lines and bridges that were key to the British military supply lines.
During World War I and World War II, 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 in anticipation of a German invasion, they were often hexagonal in shape and were called "pillboxes". About 28,000 pillboxes and other hardened field fortifications were constructed of which about 6,500 still survive.
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 in Berlin-Schöneberg has a post-war block of flats built over it. During the Cold War the shelter was in use as a NATO foodstore.
In the Guerrilla Phase of the Irish Civil War (1922-23) - a network of blockhouses was constructed to protect the railways from guerrilla attacks.
Blockhouses and Sangars have become a feature of the 2006 conflict in Afghanistan, being used by the British coalition forces, amongst others, as strong points to control the contested Southern provinces. These positions have served to draw out the Taliban, who have taken to attacking repeatedly in numbers.
The reinforced building(s) used to protect personnel and control equipment near rocket launch pads are also called blockhouses, in analogy to the military structure.
BLOCKHOUSE, in fortification, a small roofed work serving as a fortified post for a small garrison. The word, common since 1500, is of uncertain origin, and was applied to what is now called a fort d'arret, a detached fort blocking the access to a landing, channel, pass, bridge or defile. The modern blockhouse is a building, sometimes of two storeys, which is loopholed on all sides, and not infrequently, in the case of two-storey blockhouses, provided with a mdckicoulis gallery. Blockhouses are built of wood, brick, stone, corrugated iron or any material available. During the South African War (1899-1902) they were often sent from England to the front in ready-made sections.