In 1543 the English King Henry VIII'and his ally Emperor Charles V'agreed to invade France in 1544. England was at open war with France and although the intention was to mount an offensive on the French mainland, there was a risk that the French would retaliate by attacking the English coast.
The waters around the Isle of Wight and the approach to Southampton had been strengthened during the 1539 invasion crisis with the construction of number of small forts and blockhouses in the Solent, but the important town of Portsmouth was left relatively unprotected.
Even as plans were being made for the English expedition to capture Boulogne, Henry was ordering the construction of a new fort guarding the deep water channel used by ships approaching Portsmouth from the east, built at the point where this channel comes closest to the mainland.
This new fort, called Southsea Castle, represented a shift in ideas from the earlier device forts (such as Deal Castle and Camber Castle) because of the use of angled walls instead of concentric rounded gun platforms, which had a poor flanking capacity.
Although the castle did not employ the Italian bastion'system that was spreading all over the continent, it is likely that this was its inspiration. The king himself, who seems to have been at least partially responsible for the castle's design, had a keen interest in military affairs and fortification.
The original Southsea Castle took the form of a central square keep with an outer wall that had two redans'pointing south (towards the sea) and north (pointing inland). The courtyard around the keep was divided by four walls to confuse attackers.
Despite the lack of proper bastions, the two redans and the adjacent walls were flanked by guns firing through embrasures'at the foot of the wall, added at the king's own instigation in 1545. The only walls left unflanked were the end walls of the east and west wings facing up and down the coast, which were unlikely to face an attack.
The purpose of the castle was to prevent enemy ships from approaching Portsmouth and the seaward-facing ordinance was sited in the keep and on the wings to the east and west of the keep. There was a lookout tower on top of the keep for spotting enemy ships.
Southsea Castle was completed in haste and construction lasted just 6 months, spurred on by the continuing threat of a French attack. These fears turned out to be well grounded: Although Boulogne fell to the English in September 1544, Charles V made peace with France, giving the French a free hand to strike back at Henry. In July 1545 a French fleet sailed into the Solent and landed troops on the Isle of Wight. During a naval battle in front of Southsea Castle, Henry VIII witnessed the sinking of his beloved flagship, the Mary Rose.
The French retreated after the battle and the flanking improvements suggested by Henry were carried out the following year. The castle was garrisoned throughout the rest of the century but did not see action until the Civil War'in the 17th century. When the war broke out in 1642, Portsmouth and Southsea Castle were held by the Royalists, but a Parliamentarian army soon arrived and the castle, which was only garrisoned by 12 men, was taken in a surprise attack.
During the second half of the 17th century the defences of Portsmouth were rebuilt by Sir Bernard de Gomme, who also inspected Southsea Castle and recommended improvements.
One of his proposals (see map right) involved surrounding the castle with a covered way'and glacis to protect the ditch. This covered way was to have a 30 gun battery facing the sea, giving the castle a much-needed boost in coastal firepower.
It is unclear how much of de Gomme's proposal was carried out, but it is known that a programme of maintainence was undertaken at the castle and that the gateway was rebuilt with Charles II's coat of arms above it. It seems that the glacis and covered way were made and at least part of the coastal gun battery was built, although the latter was eroded away by the sea over the following decades.
Throughout the 18th century the castle was manned, but it gradually declined. In 1759 a large amount of stored gunpowder in the east wing, resulting in 17 deaths and damage to the castle. Towards the end of the 18th century fresh fears of a French invasion led to concerns about the castle's effectiveness.
However nothing was done until 1813, when the wars with France were nearly over. At this time Major-General Fisher, commander of the Royal Engineers in Portsmouth, modified the old Henrican castle to bring it up to date.
Firstly, Fisher moved the landward wall of the castle 9 metres inland, creating more space in the courtyard and transforming the redan into a proper bastion. This extra space inside the castle was used to build rooms for storage and accommodation.
The revamped north bastion could now mount up to 10 32-pounder guns, compared to the 4 guns covering the original redan. The watchtower was removed from the top of the keep so that four 24-pounders could be mounted there. On the south side of the castle, a powder magazine'was built inside the redan, which was rounded off. The seaward front of the castle, not including the keep, would have mounted 10-15 guns after the modifications.
Another important addition was the counterscarp gallery'that ran along the outer edge of the ditch, allowing the defenders to fire at any attackers in the ditch. The counterscarp gallery was accessed through a tunnel under the ditch called a caponnier.
The caponnier ran across the ditch at the south bastion (the rounded redan facing the sea) and the counterscarp gallery ran round the circuit of the ditch, with the exception of the stretch in front of the north bastion.
Major-General Fisher's improvements were completed in 1816. This made Southsea Castle ready to face Napoleonic warfare, but the first half of the 19th century saw advances in artillery that were in danger of rendering the castle obselete once more.
1850 saw the replacement of the old guns on the sea front with 7 8-inch on traversing carriages behind a new brick parapet. In the 1860s the castle was expanded by the construction of the east and west batteries either side of it so that by 1886 there were 25 rifled muzzle-loading guns at Southsea. The west battery was linked to the counterscarp gallery by the construction of a spiral staircase.
Advances in artillery technology were so fast that these guns were out of date after just a few years and had to be replaced at the turn of the century by 9.2-inch breech loading guns.
Southsea Castle remained in use through both world wars and was garrisoned until 1960, when it was sold to the council and restored.
Visiting Southsea Castle
Southsea Castle was restored to its 19th century state and today it is open to the public between April and September. The castle is in good condition and the seaward-facing sections of the east and west batteries have survived intact.
The many different stages of improvement make the visit to the castle an interesting one, with 16th, 17th and 19th century fortification elements all present in one small fort.
The caponnier and a section counterscarp gallery can be accessed. A museum in the keep explains the history of the castle from its construction by Henry VIII to the modern day. Also of interest is the collection of artillery at the castle, ranging from Tudor pieces to 19th century rifled muzzle-loaders.
The stations of Fratton, Portsmouth & Southsea and Portsmouth Harbour are all roughly 2 miles away from Southsea Castle. The walk from Portsmouth Harbour will take you past the remains of the fortifications of Portsmouth, adjacent to the seafront. Other nearby fortifications include Gosport and Carisbrooke Castle. There are various other device forts built by Henry VIII in the area, such as Hurst Castle, Calshot Castle and Yarmouth Castle.