Blockhouse 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 embrasure s 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 hexagon al or octagon al , 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 " pillbox es " . 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 tower s . 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 . Non - military . The reinforced building ( s ) used to protect personnel and control equipment near rocket launch pad s are also sometimes referred to as " blockhouses " , in analogy to the military structure .