It formed part of the King's Device programme to protect against invasion from France and the Holy Roman Empire, and defended the Solent and the eastern approach to Portsmouth.

The castle had a square central keep, two rectangular gun platforms to the east and west, and two angled bastions to the front and rear, and was an early English example of the trace italienne-style of fortification popular on the Continent.

In the interwar years some of the fortifications were stood down, but the castle saw service again in the Second World War, when it was involved in Operation Grasp, the seizure of French naval vessels in Portsmouth harbour.

Southsea 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.

[2] 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] 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.

[19] The French invasion emerged in 1545, when Admiral Claude d'Annebault crossed the Channel and arrived off the Solent with 200 ships on 19 July, landing troops on the Isle of Wight.

[20] Henry's fleet made a brief sortie, resulting in the Battle of the Solent, in which the English flagship, the Mary Rose, was lost, before retreating safely behind the protective fortifications.

[22] The French expedition moved further on along the coast on 25 July, bringing an end to the immediate invasion threat, but Henry gave orders for additional improvements to the fortification to be made that summer, including the construction of stone flankers and timber caponiers, to guard against a potential infantry attack.

[28] Southsea Castle was held by the Royalist Captain Challoner, supported by a small garrison of eleven men and 14 guns, and Norton decided to attack the fortification with 400 infantry and two troops of cavalry.

[29] The assault took place in the early hours of the morning on 5 September; the garrison had positioned their guns to point inland, so the attackers stormed the moat on the seaward side.

[30] Concerns about a potential Dutch attack grew, however, and in 1665 King Charles II commissioned his Dutch-born master engineer, Sir Bernard de Gomme, to carry out a major scheme to improve the defences of Portsmouth, including Southsea Castle.

[36] George Legge, the Baron Dartmouth, pressurised FitzJames to hand over control of Southsea Castle to the Protestant Captain Carter on 18 December and Portsmouth surrendered to the revolution two days later.

[39] The Master–General of the Ordnance, Charles Lennox, the Duke of Richmond, reported in 1785 that it was "of too bad a form to deserve the expense necessary to repair it" and proposed building a more modern fort along the coast instead.

[47] The lighthouse was in continuous use until 2017, when it was superseded by a new structure as part of changes made to approaches to Portsmouth Harbour in preparation for the arrival of the Royal Navy's new Queen Elizabeth-class aircraft carriers.

Naval gunboats launched a mock attack on the castle, which responded by firing a volley of forty 8-inch guns, causing chaos among the crowds watching around the base of the fortification.

[54] 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 expressing fears about the security of Portsmouth in 1860.

[58][b] Meanwhile, in 1785, the government had taken possession of Southsea Common, the marshland surrounding the castle, to ensure it remained open and accessible to the fort's guns, and convicts were deployed to fill in and flatten out the marshes from 1831 onwards.

[40] The surrounding area began to develop as an urban centre; seaside facilities were established at the start of the century and in the 1840s new private housing was built around the edge of the common, used by military and naval officers.

[68] During the war, the castle was manned by a range of units including coastal artillery from the regular army and the home guard, and the east battery was armed with two 9.2-inch BL Mark X guns.

[68][b] The council carried out restoration work and archaeological surveys, removing many of the later, post-1850 features and landscaping the surrounding area; in 1967 it was opened as a museum, although the lighthouse remained in operation.