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. 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.
In 1533, Henry broke with Pope Paul III over the annulment of his long-standing marriage to Catherine of Aragon. Catherine was the aunt of Charles V, the Holy Roman Emperor, who took the annulment as a personal insult. This resulted in France and the Empire declaring an alliance against Henry in 1538, and the Pope encouraging the two countries to attack England. An invasion of England appeared certain. 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. The immediate threat passed, but resurfaced in 1544, with France threatening an invasion across the Channel, backed by her allies in Scotland. Henry therefore issued another device to further improve the country's defences, particularly along the south coast.
The castle was built on the southern end of Portsea Island to protect a deep-water channel running through the Solent to the royal naval base at Portsmouth. Work began in early 1544, under the overall direction of Sir Anthony Knyvett, the Governor of Portsmouth, supported by Richard Cawarden, the Dean of Chichester, and John Chatterton, the captain of the Porstmouth garrison; Thomas Bertie was appointed as the master mason. It is uncertain who designed the castle, although Knyvett described it as being "of his Majesty's own device", which typically indicated that the King had taken a personal role. The design abandoned the earlier use of semi-circular bastions, which could not be fully covered by flanking fire from the supporting walls, and instead used an angular design, forming an early, if imperfect, adoption of the trace italienne-style of fortification already in use in Continental Europe.
The work was carried out quickly due to the risk of a French attack, and by July two brass saker guns were mounted on the site. It cost at least £3,100, £1,300 of which came from the proceeds of the Dissolution of the Monasteries, with chalk, stone and timber being brought across from the neighbouring Isle of Wight. The castle was completed by October and formed a fortification with a square keep, rectangular gun platforms to the east and west, and angled bastions to the front and rear. Chatterton was appointed the new captain, with a team of eight soldiers, twelve gunners and a porter. The castle was well-armed, with seven brass artillery pieces - a culverin, demi-culverin, cannon and demi-cannon - and eight iron guns.
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. 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. Henry was present in Portsmouth at the time and the Cowdray engraving of the battle depicts him visiting Southsea Castle. 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. Edward VI spent a night at the castle in 1552 while inspecting the defences of Portsmouth.
In the early 1600s, England was at peace with France and Spain and as a result the coastal defences received little attention; a survey found Southsea Castle to be "verie ill prepared for defence", with no guns mounted along the walls or any stocks of powder. In March 1626, a serious fire broke out that destroyed the interior of the keep and generated panic among the ships moored nearby. The damage was not repaired until 1635, when 210 loads of timber was sent from the New Forest for use in the work. Another serious fire then broke out in 1640, causing damage to the lodgings and store rooms.
At the start of the English Civil War in 1642, fighting broke out around Portsmouth between the Royalist forces, led by Colonel George Goring, and Parliamentarians under the command of Colonel Richard Norton. 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. The assault took place at 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. They then demanded that the garrison surrender but Challoner, who had been drinking heavily with Goring the night before, refused to discuss this and asked them to return later in the day. A fresh assault began and the garrison surrendered without loss of life. The capture of the castle meant left Portsmouth's position untenable and the town surrendered. A significant Parliamentarian garrison was then installed in the castle to keep it from being retaken.
The castle was primarily used as a prison during the interregnum and the early days of the Restoration. 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. Funding and poor organisation meant that the work did not begin until the 1680s. It included constructed an earthwork glacis around the castle; a new castle gate; four projecting turrets; and a redesign of the central keep. A large, stone plaque above the gatehouse entrance, bearing Charles' coat of arms, is dated 1683.
When opposition grew in 1688 against the Catholic King James II, the military establishment around Portsmouth was controlled by the King's illegitimate son, James FitzJames, the Duke of Berwick. Revolution broke out and for a period Portsmouth and its defences were important to the King as a potential exit route for the Royal Family to France; by late December, however, this route was no longer needed. 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. The traveller Celia Fiennes, who visited the castle in 1690, observed that it was "very fine, but I think it is but of little strength and service."
The east side of the castle was badly damaged in an explosion in August 1759, caused after cooking sparks fell onto gunpowder stored in the castle by the 72nd Regiment of Foot; 17 men, women and children were killed. Probably as a consequence of this event, a new powder store called the Firebarn was built outside the north side of the castle. By the second half of the century, the castle was in poor condition and, according to one contemporary account, only garrisoned by "an old sergeant and three or four men who sell cakes and ale". 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. By the end of the century, coastal erosion had destroyed most of the 18th-century grand battery.
With the outbreak of the Revolutionary and Napoleonic Wars at the end of the 18th century, Southsea Castle's importance increased once again. In 1797, a French invasion appeared imminent and the castle was urgently readied for action; only eight 32-pounder (1.45 kg) and five 6-pounder (2.7 kg) guns were available and these were in a poor condition. In 1813, work was begun to expand the castle, under the direction of Major-General Benjamin Fisher. Fisher had the work carried out by a mixture of soldiers and civilian contractors, arguing that it made it easier to contain costs and prevent union activity and the project cost an estimated £18,105. The fort was extended north by up to 30 feet (9.1 m), with the keep and interior bailey redesigned in brick. The moat was rebuilt and a counterscarp galley was built round the edge. Most of the work was finished by 1814 although the final elements were not completed until 1816.
A lighthouse, commissioned by the Admiralty, was constructed on the western gun platform in 1828, rising 34 feet (10 m) above the its base atop the walls. The lighthouse keeper was housed in the castle itself. An 1837 enquiry had looked into the management of military offenders in the south of England and in 1844 it was decided to bring Southsea Castle and Fort Clarence into use as military prisons, to reduce the pressure on the civilian gaols in the area and to provide a more suitable military environment for the prisoners. Southsea was used as a prison until 1850, holding 150 offenders under the supervision of a Royal Artillery sergeant who also oversaw the remaining artillery defences.
The introduction of shell guns and steam ships in the 1840s created a new risk that the French might successfully attack along the south coast. In 1850, with the closure of the prison, seven 8-inch (22.3 cm) guns were mounted along the walls in brick emplacements, but there were concerns about the defences and two earthwork auxiliary batteries were built just alongside the castle to house additional guns. In 1856, at the end of the Crimean War, there was a large review of the fleet along the Solent, attended by Queen Victoria and numerous tourists. 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.
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. Lieutenant-Colonel Jervois oversaw the broader programme of work across the country, and was supported at Southsea by Lieutenant-Colonel Fisher. New gun batteries with underground magazines were constructed on the west and east sides of the castle between 1863 and 1869 to form part of a 17-acre (6.9 ha) complex enclosed by a defensive wall; the old castle was used for range and direction finding, rather than to house weapons, and was left to slowly decline. By 1886, the site was well-armed, with 25 rifled, muzzle-loading guns and the design was praised by a visiting Russian military engineer.
By the 1890s, the castle's defences had become obsolete once again due to the development of torpedo boats and superior breech-loading guns. In response, five 6-pounder quick-firing guns and searchlights were installed to deal with the torpedo boat threat and, between 1899 and 1901, the east battery was converted to hold breech-loading guns at a cost of £16,670.
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. 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. The construction of a railway line from London enabled the settlement of Southsea, named after the castle, to turn into a prosperous resort. Southsea Common was leased by the local government in 1884 and, as the population grew, practising firing the castle's guns became more difficult.
At the start of the 20th century, Southsea Castle formed part of the "Fortress Portsmouth" plan for defending the Solent, the east battery equipped with two BL 6-inch (15.2 cm) Mark VII guns and two BL 9.2-inch (23.3 cm) Mark X guns, and the west battery armed with three 12-pounders (5.4 kg) and one 4.7-inch (11.9 cm) QF gun. During the First World War, the castle was at first manned by Royal Garrison Artillery and Number 4 company of the Hampshire Royal Garrison Artillery Territorials. Later these units were transferred to France and were replaced by the Hampshire Royal Garrison Artillery Volunteers. In response to the threat from German zeppelin raids, a QF 3-inch 20 cwt (10.9 cm 50.8 kg) anti-aircraft gun was added to the castle. Even though the castle remained in active military use after the conflict, by 1929 it had become a tourist attraction, and visitors were able to watch the castle garrison carry out practice firings out to sea. Southsea Common was bought by Portsmouth City Council in 1922 and turned into a park, while the west battery was disarmed in 1927.
During the Second World War, the Hampshire Heavy Regiment, Royal Artillery (Territorial Army), occupied the castle, which was used as the Headquarters Portsmouth Fixed Defences. 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. Living conditions were poor, with the keep described by one member of the Auxiliary Territorial Service as "cold and wet and horrible". The castle was protected by barrage balloons, but it was hit by at least two incendiary bombs which did little damage.
During the war, the castle was involved in an armed stand-off with French naval vessels. After the Fall of France in 1940, some of the French ships had made their way to British ports, including Portsmouth, but fears rose that the warships might leave and fall into German hands. On 23 June, the castle was ordered to man its guns and to be prepared to fire on the French navy to prevent their departure; one gun, albeit not ready for action, was levelled at the fleet and the French destroyer Léopard responded by aiming its guns at the castle, but no shooting ensued. On 3 July, under Operation Grasp, British forces boarded and seized the ships, ending the confrontation.
Southsea Castle was obsolete in the post-war years and in 1960 it was sold to Portsmouth City Council for £35,000. 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. The west and east batteries partially demolished by the council in the 1960s.
In the 21st century the castle is still operated as a tourist attraction by the council and received over 90,000 visitors from 2011–12. The castle houses a collection of cannon. Two of these, a 68-pounder (30.8 kg) and an RML 9-inch 12 ton (22.8 cm 12,192 kg) gun, are location in the grounds, and within the castle itself is a 24-pounder (10.8 kg) from HMS Royal George, an RML 9-pounder 8 cwt (4 kg 406 kg) and two hexagonally rifled Witworth 3-pounder (1.3 kg) breech-loaders dating from 1876. The castle is protected under UK law as a Scheduled Ancient Monument.