[a] Traditionally, the Crown had left coastal defences in the hands of local lords and communities but the threat of French and Spanish invasion led the King to issue an order, called a "device", for a major programme of work between 1539 and 1547.

The fortifications ranged from large stone castles positioned to protect the Downs anchorage in Kent, to small blockhouses overlooking the entrance to Milford Haven in Pembrokeshire, and earthwork bulwarks along the Essex coast.

Most of the fortifications saw service in the First and Second English Civil Wars during the 1640s and were garrisoned during the Interregnum, continuing to form the backbone of England's coastal defences against the Dutch after Charles II was restored to the throne in 1660.

Fears over a possible French invasion resurfaced several times in the 19th century, combined with rapid changes in technology, such as the development of steamships and shell guns in the 1840s, rifled cannon and iron-clad warships in the 1850s, and torpedo boats in the 1880s.

[10] His son, Henry VIII inherited the throne in 1509 and took a more interventionist approach in European affairs, fighting one war with France between 1512 and 1514, and then another between 1522 and 1525, this time allying himself with Spain and the Holy Roman Empire.

[28] Work was also begun on Calshot Castle in Fawley and the blockhouses of East and West Cowes on the Isle of Wight to protect the Solent, which led into the trading port of Southampton.

[31] Carrick Roads was an important anchorage at the mouth of the River Fal and the original plans involved constructing five new fortifications to protect it, although only two castles, Pendennis and St Mawes, were actually built, on opposite sides of the estuary.

[51] The efforts of the Hampton Court Palace architectural team, under the leadership of the Augustinian canon, Richard Benese, contributed to the high-quality construction and detailing seen in many of Henry's Device projects.

[56] One of these foreign engineers, Stefan von Haschenperg from Moravia, worked on Camber, Pendennis, Sandgate and St Mawes, apparently attempting to reproduce Italian designs, although his lack of personal knowledge of such fortifications impacted poorly on the end results.

[60] Medieval castles were private dwellings as well as defensive sites, and usually played a role in managing local estates; Henry's forts were organs of the state, placed in key military locations, typically divorced from the surrounding patterns of land ownership or settlements.

[75] The multiple tiers of guns gave the forts a relatively high profile, exposing them to enemy attack, and the curved surfaces of the hollow bastions were vulnerable to artillery.

[70] It is uncertain how far the guns of the period would have reached; analysis carried out in the 16th and 17th century on the ranges of artillery suggested that the largest weapons, such as a culverin, could hit a target up to between 1,600 and 2,743 metres (5,249–8,999 ft) away.

[129] As part of this work, West Tilbury was brought back into service and supported by a hastily raised army, which was visited by Queen Elizabeth I, and further enlargement followed under the direction of the Italian engineer, Federigo Giambelli.

[134] The fleet was forced to turn back due to bad weather, but Elizabeth reviewed the defences and significantly expanded Henry's original fortifications with more up-to-date bastions, designed by the engineer Paul Ive.

[138] Castles such as Deal and blockhouses like Gravesend were all assessed as needing extensive repairs, with Sandgate reported to be in such a poor condition that "neither habitable or defensible against any assault, nor any way fit to command the roads".

[141] Some of the forts fell out of use; Camber Castle, whose original function of protecting the local anchorage had by now been made redundant by the changing shoreline, was decommissioned by King Charles I in 1637, while Sharpenrode Bulwark lay in ruins by the 1620s.

[179] Deal continued to play an important role in defending the Downs during the Second and Third Dutch Wars, supported by local trained bands, and castles such as Hurst, Portland and Sandgate remained garrisoned.

[190] There was probably some rivalry between Sackville and the naval officer Sir John Norris, who redeveloped nearby Deal Castle during the same period, creating comfortable wood-panelled quarters for himself there overlooking the sea.

[191] Portland suffered badly from coastal erosion and, protected only by a caretaker garrison, was reportedly not repaired for many years, and a 1714 survey found the long-neglected Pendennis Castle to be "in a very ruinous condition".

[202] Walmer Castle was used by its captain William Pitt the Younger – then both prime minister and Lord Warden of the Cinque Ports – as the base for volunteer cavalry unit and a fleet of 35 armed fishing boats called luggers.

[207] Calshot was a good location for interception vessels to lie in wait and, by the middle of the century, two officers and forty-two men were stationed there; Sandown Castle in Kent was also used by the coastguard for anti-smuggling operations.

[218] Fresh worries about France, combined with the development of rifled cannon and iron-clad warships, led to the Royal Commission on the Defence of the United Kingdom being established in 1859, and expressing fears about the security of the south coast.

[227] The original 16th-century parts of fortifications such as Southsea and Calshot were too small and unsuitable for modern weapons, however, and were instead used for mounting searchlights, range and direction finding; in some cases their fabric was left to slowly decline.

[231] A 1905 review of the Falmouth defences concluded that the naval artillery at St Mawes had become superfluous, as the necessary guns could be mounted at combination of Pendennis and newer sites along the coast, and the castle was disarmed.

[236] Portland Castle was placed onto what was known as the Schedule C list, which meant that the Army would continue to use and manage the historic property, but would receive advice on the suitability of repairs from the Office of Works.

[240] St Mawes and Portland were used as barracks, and Walmer became a weekend retreat for the Prime Minister, Asquith, exploiting its good communication links with the front line in France.

[241] During the Second World War, Britain's coastal defences depended on extensive barriers constructed along the shores, combined with large numbers of small defensive artillery positions supported by air cover.

[244] Others were used as support facilities; Yarmouth was requisitioned for military use; Portland was used for accommodation, offices and as an ordnance store, and West Cowes used as a naval headquarters for part of the D-Day landings.

[247] The Prime Minister, Winston Churchill, was appointed as the captain of Walmer Castle during the war, but declined to use it as a residence, noting that it was too expensive for him to maintain, and that it lay within the range of German artillery.

[251] Widespread restoration work was then carried out; at Calshot, Deal, Hurst, Pendennis, Portland, St Catherine's and Southsea, the more modern additions to the fortifications were destroyed in an attempt to recreate the appearance of the castles at earlier periods of their history, ranging from the 16th to the 19th centuries.