Coastal defence and fortification Coastal defence ( " Commonwealth and Ireland " ) or coastal defense ( " US " ) and coastal fortification are measures taken to provide protection against attack by military and naval forces at or near the shoreline . Because an invading enemy normally required a port or harbour to sustain operations , such defences are usually concentrated around such facilities , or places where such facilities could be constructed . Sea forts . Although most coastal fortifications are on the coast , not all are . Instead , some are off the coast on islands , artificial islands , or are specially built structures . All the sea forts share the characteristic that at least at high tide they are completely surrounded by water . Some , such as Fort Denison or Fort Sumter , are actually in harbours , but most are off the coast . Again , some , such as for example Bréhon Tower , completely occupy small islands , although others , such as for example Flakfortet and Pampus , are on artificial islands built up on shoals . Fort Louvois is on a built - up island , 400 meters from the shore , and connected to it by a causeway that high tide completely submerses . The most elaborate sea fort is Murud - Janjira , which is so extensive that one might truly call it a sea fortress . The most recent sea forts were the Maunsell Forts , which the British built during World War II . One type consisted of a concrete pontoon barge on which stood two cylindrical towers on top of which was the gun platform mounting . They were laid down in dry dock and assembled as complete units . They were then fitted out before being towed out and sunk onto their sand bank positions in 1942 . The other type consisted of seven interconnected steel platforms built on stilts . Five platforms carried guns arranged in a semicircle around the sixth platform , which contained the control centre and accommodation . The seventh platform , set further out than the gun towers , was the searchlight tower . Coastal defence and fortification by country . China . China first established formal coastal defences during the early Ming dynasty ( 14th century ) to protect against attacks by pirates ( wokou ) . Coastal defences were maintained through both the Ming dynasty and the Qing dynasty that followed , protecting the coast against pirates , and against the Portuguese and other European powers that sought to impose their will on China . Subsequently , the European powers built their own coastal defences to protect the various colonial enclaves that they established along the Chinese coast . One such , a fort built by the British commanding the Lei Yue Mun channel between Hong Kong Island and the mainland , has been converted into the Hong Kong Museum of Coastal Defence . This tells the story of coastal defence along the South China coast from the Ming dynasty onwards . New Zealand . The coastline of New Zealand was fortified in two main waves . The first wave occurred around 1885 and was a response to fears of an attack by Russia . The second wave occurred during World War II and was due to fears of invasion by the Japan ese . The fortifications were built from British designs adapted to New Zealand conditions . These installations typically included gun emplacements , pill boxes , fire command or observation post s , camouflage strategies , underground bunker s , sometimes with interconnected tunnels , containing magazines , supply and plotting rooms and protected engine rooms supplying power to the gun turret s and searchlights . Taiwan . Taiwan has several coastal fortifications , with some , such as Fort Zeelandia or Anping Castle dating to the time of the Dutch East India Company . Others , such as Cihou Fort , Eternal Golden Castle , Hobe Fort , date more to the end of the 19th Century . The Uhrshawan Battery dates primarily to the first - half of the 19th Century . It actually underwent bombardment during the Sino - French War . United States . The defence of its coasts was a major concern for the United States from its independence . Prior to the American Revolution many coastal fortifications already dotted the Atlantic coast , as protection from pirate raids and foreign incursions . The Revolution led to the construction of many additional fortifications , mostly comprising simple earthworks erected to meet specific threats . The prospect of war with European powers in the 1790s led to a national programme of fortification building spanning seventy years in three phases , known as the First , Second and Third Systems . By the time of the American Civil War , advances in armour and weapon s had made masonry forts obsolete , and the combatants discovered that their steamships and ironclad warship s could penetrate Third System defences with acceptable losses . In 1885 US President Grover Cleveland appointed the Endicott Board , whose recommendations would lead to a large - scale modernization programme of harbour and coastal defences in the United States , especially the construction of well dispersed , open topped reinforced concrete emplacements protected by sloped earthworks . Many of these featured disappearing guns , which sat protected behind the walls , but could be raised to fire . Mine fields were a critical component of the defence , and smaller guns were also employed to protect the mine fields from minesweeping vessels . The development of military aviation rendered these open topped emplacements vulnerable to air attack . Therefore , the next , and last , generation of coastal artillery was mounted under thick concrete shields covered with vegetation to make them virtually invisible from above . In anticipation of a conflict with Japan , most of the limited funds available between 1933 and 1938 were spent on the Pacific coast . In 1939 the threat of war in Europe prompted larger appropriations and the resumption of work along the Atlantic coast . United Kingdom . The walls around coastal cities , such as Southampton had evolved from simpler Norman fortifications by the start of the 13th century . Later , King Edward I was a prolific castle builder and sites such as Conwy Castle , built 1283 to 1289 , defend river approaches as well as the surrounding land . Built 1539 to 1544 , the Device Forts are a series of artillery fortifications built for Henry VIII to defend the southern coast of England . Between 1804 and 1812 the British authorities built a chain of towers known as Martello Tower s to defend the south and east coast of England , Ireland , Jersey and Guernsey against possible invasion from France . The Palmerston Forts are a group of forts and associated structures built during the Victorian period on the recommendations of the 1860 Royal Commission on the Defence of the United Kingdom , following concerns about the strength of the French Navy . In 1865 Lieutenant Arthur Campbell Walker , of the School of Musketry advocated the use of armoured train s on " an iron high - road running parallel with that other ' silent highway ' , the source of all our greatness , the ocean , our time - honoured ' moat and circumvallation ' " During the First World War the British Admiralty designed eight towers code named M - N that were to be built and positioned in the Straits of Dover to protect allied merchant shipping from German U - boat s . Nab Tower is still in situ . The Maunsell Forts were small fortified towers built in the Thames and Mersey estuaries during the Second World War