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. The Admiralty Citadel in London is one of the sturdiest 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)