Coastal defence (or defense) and coastal fortification are measures taken to provide protection against military attack at or near a coastline (or other shoreline), for example, fortification and coastal artillery. Because an invading enemy normally requires a port or harbour to sustain operations, such defences are usually concentrated around such facilities, or places where such facilities could be constructed.
Rather than the beach assault of modern amphibious operations, seaborne assaults of the classical and medieval age more often took the form of raiders sailing up river and landing well inland of the coast. Prior to the invention of naval artillery that could sink hostile ships, the most that coastal defence could do was act as an early warning system, that could alert local naval or ground forces of the impending attack. For example, in the late Roman period the Saxon Shore was a system of forts at the mouths of navigable rivers, and watch towers along the coast of Britannia and Gaul.
Later in Anglo-Saxon Wessex protection against Viking raiders took the form of coast watchers whose duty was to alert the local militia; the navy, which would attempt to intercept the raider's ships or failing that to destroy them after they had beached, against smaller raiding forces the threat of losing their ships, and their way home with their loot was often enough to force them to curtail their attack; and a system of fortified towns, these burghs were positioned at choke points along navigable rivers to prevent raiders from sailing inland.
Sea forts are completely surrounded by water – if not permanently, then at least at high tide (i.e. they are tidal islands).
Unlike most coastal fortifications, which are on the coast, sea forts are not. Instead, they are off the coast on islands, artificial islands, or are specially built structures. Some sea forts, such as Fort Denison or Fort Sumter, are within harbours in proximity to the coast, but most are at some distance off the coast. Some, such as for example Bréhon Tower or Fort Drum completely occupy small islands; others, such as Flakfortet and Pampus, are on artificial islands built up on shoals. Fort Louvois is on a built-up island, 400 meters (1,312 ft) 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.[according to whom?]
The most recent sea forts were the Maunsell Forts, which the British built during World War II as anti-aircraft platforms. 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.
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.
The islands of Malta, Gozo and Comino all have some form of coastal fortification. The area around the Grand Harbour was possibly first fortified during Arab rule, and by the 13th century, a castle known as the Castrum Maris was built in Birgu to protect the harbour. The Maltese islands were given to Order of Saint John in 1530, who settled in Birgu and rebuilt the Castrum Maris as Fort Saint Angelo. In the 1550s, Fort Saint Elmo and Fort Saint Michael were built, and walls surrounded the coastal cities of Birgu and Senglea. In 1565, the Great Siege of Malta reduced many of these coastal fortifications to rubble, but after the siege they were rebuilt. The fortified city of Valletta was built on the Sciberras Peninsula, and further modifications were made to the fortifications over the years. The harbour area was strengthened even more by the building of the Floriana Lines, Santa Margherita Lines, Cottonera Lines and Fort Ricasoli in the 17th century and Fort Manoel and Fort Tigné in the nearby Marsamxett Harbour in the 18th century. The Order also built Fort Chambray near Mġarr Harbour in Gozo.
In the early 15th century, a number of watch posts had been established around Malta's coastline. In the early 17th century, the Order began to strengthen the coastal fortifications outside the harbour area, by building watchtowers. The first of these was Garzes Tower, which was built in 1605. The Wignacourt, Lascaris and De Redin towers were built over the course of the 17th century. The last coastal watchtower to be built was Isopu Tower in 1667. Between 1605 and 1667, a total of 31 towers were built, of which 22 survive today (with another 3 in ruins).
From 1714 onwards, about 52 batteries and redoubts, along with several entrenchments, were built around the coasts of Malta and Gozo. Many of these have been destroyed, but a few examples still survive.
After the British took Malta in 1800, they modified the Order's defences in the harbour area to keep up with new technology. The Corradino Lines were built in the 1870s to protect the Grand Harbour from landward attacks. Between 1872 and 1912, many forts and batteries were built around the coastline. The first of these was Sliema Point Battery, built to protect the northern approach to the Grand Harbour. A chain of fortifications, including Fort Delimara and Fort Benghisa, was also built to protect Marsaxlokk Harbour.
From 1935 to the 1940s, the British built many pillboxes in Malta for defence in case of an Italian invasion.
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 Japanese.
The fortifications were built from British designs adapted to New Zealand conditions. These installations typically included gun emplacements, pill boxes, fire command or observation posts, camouflage strategies, underground bunkers, sometimes with interconnected tunnels, containing magazines, supply and plotting rooms and protected engine rooms supplying power to the gun turrets and searchlights.
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.
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 weapons had made masonry forts obsolete, and the combatants discovered that their steamships and ironclad warships 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. Underwater mine fields were a critical component of the defence, and smaller guns were also employed to protect the mine fields from minesweeping vessels. In 1901 the Artillery Corps was divided into field artillery and coast artillery units, and in 1907 the United States Army Coast Artillery Corps was created to operate these defences.
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 1939–40 the threat of war in Europe prompted larger appropriations and the resumption of work along the Atlantic coast. Under a major program developed in the wake of the Fall of France in 1940, a near-total replacement of previous coast defenses was implemented, centered on 16-inch guns in new casemated batteries. These were supplemented by 6-inch and 90 mm guns, also in new installations.
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 Towers to defend the south and east coast of England, Ireland, Jersey and Guernsey against possible invasion from France. This type of tower was also used elsewhere in the British Empire and in the United States. 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 trains 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-boats. Nab Tower is still in situ. The Maunsell Forts were small fortified towers, primarily for anti-aircraft guns, built in the Thames and Mersey estuaries during the Second World War.