It formed part of the king's Device Forts coastal protection programme against invasion from France and the Holy Roman Empire, and defended the western entrance to the Solent waterway.

Technological developments rapidly made these defences obsolete, however, and a fresh phase of work between 1861 and 1874 created sixty-one gun positions in two long, granite-faced batteries alongside the older castle.

In January 2021, local media reported that the castle was in urgent need of repairs due to coastal erosion,[1] and the wall of the eastern wing partially collapsed on 26 February 2021 as the sea cut into and compromised its foundations.

[3] Hurst Castle was built as a consequence of international tensions between England, France and the Holy Roman Empire in the final years of the reign of King Henry VIII.

Traditionally the Crown had left coastal defences to local lords and communities, only taking a modest role in building and maintaining fortifications, and while France and the Empire remained in conflict, maritime raids were common but an actual invasion of England seemed unlikely.

[6] Modest defences based around simple blockhouses and towers existed in the south-west and along the Sussex coast, with a few more impressive works in the north of England, but in general the fortifications were limited in scale.

[7] Worsley's Tower, for example, built opposite the future site of Hurst Castle in the 1520s, was too small to hold powerful artillery and considered by surveyors in 1539 to be "one of the worst devised things" they had seen.

[12] In response, Henry issued an order, called a "device", in 1539, giving instructions for the "defence of the realm in time of invasion" and the construction of forts along the English coastline.

[15] It was positioned on the Hurst Spit, a strip of shingle sheltering saltmarsh and mud flats, only 0.75 miles (1.21 km) across the water from the Isle of Wight.

[16] Temporary earthwork fortifications were erected on the site and, after the other three castles had been completed, work then began on Hurst in 1541 under the direction of John Mille, the financial controller, and probably Thomas Bertie, a master mason.

[20] In 1547, however, Hurst was equipped with 26 artillery pieces–four made of brass and the remainder iron–comprising a two sakers, a culverin, a demi-cannon, a curtall cannon, two demi-culverins, six portpieces, four slings, two quarter-slings, and seven bases, three of them inoperable.

[22] The survey observed that the castle was vulnerable to attack because it lacked flanking protection and had rounded walls, and that it was expensive to garrison because of its size, requiring a captain, his deputy, twelve gunners, nine soldiers and a porter.

[22] The historian John Kenyon notes, however, that its considerable armament made it one of the most powerful forts in the south, even if it was equipped with lighter guns than would have been ideal for its "ship-killing" role.

[28] Instead of maintaining a regular garrison, in 1666 it was decided to staff the castle using soldiers deployed from the Isle of Wight instead, from a unit belonging to Sir Robert Holmes, the island's governor.

[32] Amid concerns over the moral condition of England and a perceived threat to the established Church, an act for "further preventing the growth of popery" was passed in 1700; Hurst was chosen by the Privy Council to house any priests convicted under this law.

[35] In 1729, the Revenue service hired the Hurst, a wide, heavy yacht, to assist in anti-smuggling operations out of Southampton, arming its crew with muskets, pistols and swords.

[37] The castle fell into neglect, and reports from the 1770s through into the early 1790s noted extensive problems, complaining that the fort's guns could no longer be mounted on the dilapidated bastions and that water was seeping through the decaying walls.

[42][b] The 16th-century castle was still strongly fortified by the standards of the time, but it could not support the heavier guns or batteries of artillery required to tackle enemy warships.

[46] After some discussions, it was agreed to adapt the 16th-century keep to enable it to hold six 24-pounder (10.8 kg) guns; the roof was vaulted and a central stone pillar was installed to run up through the building, work estimated at the time to be likely to cost £4,122.

[48] It was proposed to build two temporary gun batteries to replace the 1795 emplacements, which had suffered from the salt air and decayed, but the plan was turned down in order to focus attention on the redevelopment of the keep.

[52] Official interest in Hurst Castle increased again in the 1840s, as the introduction of shell guns and steam ships created a new risk that the French might successfully attack along the south coast.

[53] Previously, sailing ships had been only able to pass the castle slowly when moving against the tide, making them vulnerable to its guns; steamships threatened to cruise past at speed.

[76] The castle retained seventeen of them, as historian Coad states, wired up together "like a collection of elderly blunderbusses", but the fort depended on its newer quick-firing weapons.

[77][c] After the war, the guns were removed from the 16th-century part of the castle, which was passed into the national collection of the Ministry of Works in 1933, although some modernisation of the rest of the fortification took place in the 1930s.

[88] This erosion has increased significantly since the 1940s, due to the construction of groyne barriers at nearby Bournemouth and Christchurch which prevented the natural renewing of the spit with pebbles washed from local cliffs.

[16][89] Government efforts began in the 1960s to try to stabilise the spit in its current position, both to protect the castle and neighbouring towns, with over 900,000 tonnes (990,000 tons) of gravel being laid down in the 1990s, and is continuing in the 21st century.

[91] In January 2021, local media reported that the castle was in urgent need of repairs due to coastal erosion[1] and the wall of the eastern wing partially collapsed on 26 February 2021.

[99] Both have eight embrasure windows, suitable for holding lighter weaponry; the first-floor room was sufficiently elevated to have potentially fired out over the external walls.

[71] The West Wing is approximately 215 metres (705 ft) long, and has 37 heavy gun positions and two main magazines, along with various auxiliary buildings, including canteens, stores and detention facilities.

[107] A small theatrical theatre, built by gunners in the Second World War, survives in one of the gun positions, along with various wall paintings, possibly used in performances.