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.
Early blockhouses in England 
Early blockhouses were designed solely to protect a particular area by the use of artillery, and they had accommodation only for the short-term use of the garrison. The first known example is the Cow Tower, Norwich, built in 1398, which was of brick and had three storeys with the upper storeys pierced for six guns each. The major period of construction was in the maritime defence programmes of Henry VIII between 1539 and 1545. They were built to protect important maritime approaches such as the Thames Estuary, the Solent, and Plymouth. Often sited in pairs, the blockhouses were not built to a common design, but usually consisted of a stone tower and bastion or gun platform, which could be semi-circular, rectangular or irregular in shape. The last blockhouse of this type was Cromwell's Castle, built in Scilly in 1651.
Age of Exploration 
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.
Second Boer War 
During the Second Boer War the British forces built a large number of fortifications in South Africa. Around 441 were solid masonry blockhouses, many of which 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.
However the vast scale of British strategy led the British to develop cheaper, double-skinned corrugated iron structures. These could be prefabricated, delivered to site by armoured train, and then have locally sourced rocks or rubble packed inside the double skin to provide improved protection.
A circular design developed by Major Rice in February 1901 had good all round visibility, and the lack of corners did away with the need for a substructure. Failure due to wood rot and splintering when hit by bullets or shrapnel were eliminated. The steel door to the blockhouse was sheltered by another piece of corrugated iron. The Major Rice blockhouse could be erected in six hours by six trained men. With the change from square gabled roofs to a circular design, they were given the nickname “Pepperpot blockhouse With mass production the cost to build a blockhouse dropped down to £16, compared to several hundred pounds for masonry ones.
These blockhouses played a vital role in the protection of the railway lines and bridges that were key to the British military supply lines.
Concrete blockhouses 
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 (literally, "high bunkers"; better translated as 'above ground bunkers', to distinguish them from the usual deep i.e. underground air raid shelters) and those that 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.
Afghanistan 2006 
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 sometimes referred to as blockhouses, in analogy to the military structure.
See also 
- Battery tower
- Martello tower
- British hardened field defences of World War II
- Blockhouse, Nova Scotia
- Sangar (fortification)
- "Definition of a Blockhouse".
- "Blockhouse dates".
- "Blockhouse general description".
- Young, Richard (1980). Blockhouses in Canada, 1749-1841: a Comparative Report and Catalogue. Occasional Papers in Archaeology and History, Canadian Historic Site. ISBN 978-0-660-10298-6.
- Tomlinson, Richard. "Britain's Last Castles - Masonry Blockhouses of the South African War, 1899-1902". Military History Journal (The South African Military History Society) 10 (6). Retrieved 2009-02-14.
- "Blockhouses of the Boer War". Retrieved 2012-07-23.
- For the difference between blockhouses and bunkers see Schneider & Kitchen 2002, p. 87, BACM Research 2009, p. 209, Davis 2007, p. 290
- "A Review Of The Defence of Britain Project". Report. Retrieved 2006-05-30.
- "Lupin Pooter".
- BACM Research (2009), Vietnam War After Action Reports, BACM Research, pp. 263
- Davis, Tracy C. (2007), Stages of emergency: Cold War nuclear civil defense (illustrated ed.), Duke University Press, p. 290, ISBN 978-0-8223-3970-0
- Schneider, Richard Harold; Kitchen, Ted (2002), Planning for crime prevention: a transatlantic perspective, RTPI library series 3 (illustrated ed.), Routledge, p. 87, ISBN 978-0-415-24136-6
|Wikimedia Commons has media related to: Blockhouses|
- Pillbox Study Group
- Royal Engineers Museum: Blockhouses during the Anglo-Boer War (1899–1902)
- Berlin Air-raid Shelters, Flak Towers and Bunkers
- British World War 2 Fortifications
- The Fortress Study Group