In readiness for a potential invasion by the Spanish and French, as a result of Henry VIII's divorce from Catherine of Aragon, a series of coastal 'forts' were quickly erected along the vulnerable European coastline, from the River Humber in the north east to Cornwall in the south west. One of the last to be constructed was Southsea Castle, built in 1544 to protect the deep water approach into Portsmouth Harbour.
Many differences can be seen in the later design of Henry's forts. Unlike his castles at Deal and St Mawes, where the concentric circular bastions are particularly striking, the defences at Southsea are completely angular. Comprising initially of a rectangular east and west bastion, a pointed north and south bastion, and a square central keep, Southsea Castle was designed to offer the minimum target possible to enemy guns. This arrangement also ensured that Southsea Castle's gunners had a clear shot along the curtain walls, thus eliminating any 'dead' ground. The whole was then enclosed by a 16ft (5m) dry moat.
In keeping with earlier medieval castles, Southsea Castle had a central open area (between the curtain walls and the keep), called the bailey. This was divided into four compartments by passageways or 'traverses' that ran from the four corners of the keep to the curtain wall, and ensured that any attackers scaling a section of curtain wall and descending into the bailey would make for an easy infantry target. Before Henry VIII's reign came to an end, Southsea Castle was armed with a total of 17 guns.
During the remaining Tudor years and the reign of James I, Southsea Castle was slowly allowed to fall into a state of disrepair. Further dilapidated by a serious fire to the keep in 1627, Southsea Castle was eventually repaired under reluctant orders from King Charles I in 1635. With the advent of the English Civil War Southsea Castle fell into the hands of the Parliamentarians very early on, but without any the loss of life inflicted on either side. Following the restoration in 1660 a further invasion threat was posed from the Dutch and, in order to secure Portsmouth Harbour, Charles II instigated a further series of improvements and modifications to Southsea Castle. A new earthen battery was built between the dry moat and the sea to improve firepower capability, and a high sloping earthwork or 'Glacis' was constructed on the landward side to protect the curtain wall from land-based artillery.
For the next 130 years Southsea Castle was again allowed to deteriorate, aggravated on this occasion by a considerable gunpowder explosion that destroyed the eastern section in 1759. Demolition of Southsea Castle was proposed in the late 18th century to facilitate the construction of a more modern redoubt, but nothing had happened by the time of the next threat of invasion from Napoleon Bonaparte in 1804. Finally, in 1813, the long awaited renovation and modernisation plan for the ageing fortress was approved. A major programme of work was carried out, including an entire remodelling of the interior of the keep, and the removal of the watchtower to facilitate four additional gun positions; the demolition of the original north bastion, and the building of a replacement some 30ft (9m) further inland allowing for extra accommodation beneath, and greater gun capacity above; alterations to the southern seaward facing bastion to give it the unusual curved shape that can be seen today; and the construction of a counterscarp gallery in the outer southern moat wall. This was accessed by a 'Carponier' that ran across the floor of the dry moat, from the south curtain wall. Further accommodation was also created beneath the east and west bastions. At the request of the Admiralty, the lighthouse was erected near the west bastion in 1820. Additional gun batteries were added to the east and the west flanks some 40 years later, and a brick wall with loopholes was erected on the landward side to give some protection from these batteries being over run.
Southsea Castle had now been transformed from a redundant ruin to a key element in the defensive strategy for Portsmouth, and it maintained an important role throughout the Two World Wars. By the 1950s, modern weapons had rendered coastal artillery obsolete and Southesea Castle was subsequently purchased by the City Council in 1960. Within seven years Southsea Castle had been returned to its 19th century appearance, and was opened to the public for the first time in 1967.