Calshot Castle was built to protect the prosperous port of Southampton as part of Henry VIII’s large scale coastal defence programme. The site was later re-armed in the late nineteenth century in response to the new Motor Torpedo Boat threat and soon after became a major seaplane base for the Royal Navy.
Calshot Castle was constructed as part of the 1539 programme of 'Device' forts commissioned by Henry VIII to protect the South Coast from potential invasion from French or Spanish forces. The Solent was regarded as particularly vulnerable due to the access afforded to the rich town of Southampton. William Fitzwilliam, Earl of Southampton and William Paulet, Lord St John were charged with proposing defence for the Solent region and recommended four forts at Calshot, Hurst plus East and West Cowes.
Calshot Castle was built on a shingle spit that was in close proximity itself to the deep water passage into Southampton Water. By Spring 1539 work was underway and, such were the fears of invasion, an earthwork fortification was probably erected to provide defence whilst the stone castle was built. Completed by the end of 1540, the project was doubtless hastened by ample quantities of prepared stone quarried from the recently dissolved abbeys at Netley and Beaulieu. Built to a concentric design, a circular Keep was set within a low circular curtain wall. In total it had three levels of guns; one on top of the curtain wall itself, one within the Keep and one on top of the Keep roof. On completion of Calshot the team moved onto Hurst Castle whilst further fortifications - St Andrews Castle (Hamble) and Netley Castle - were added in the Solent in 1544.
The invasion fears of Henry's reign passed unrealised but the castle remained garrisoned over the subsequent years albeit the number of serviceable weapons was allowed to reduce. Serious damage was suffered from a fire that gutted the building during the Elizabethan period but repairs were made and the castle was prepared in the 1580s to defend against potential invasion by Spain.
Calshot saw no action during the seventeenth century Civil War but, unlike both Netley and St Andrew's Castles, its strategic location meant it avoided slighting by Parliamentary forces. The castle remained garrisoned and continued to perform its coastal defence role throughout the eighteenth century. Upgrades were made in 1774 including lowering the curtain wall which converted the gun positions of the original structure into open topped embrasures.
Admiral Nelson's victory at the Battle of Trafalgar (1805), followed by the Duke of Wellington's later continental victory against Napoleon, reduced the threat to mainland Britain significantly. For the next 100 years the Royal Navy would rule supreme providing protection to the country. At this time the need for coastal defence diminished and Calshot became a coastguard station tasked with reducing smuggling. This duty, along with ownership of the castle, transferred to the Royal Navy in 1856.
Calshot was not upgraded as part of the major fort building associated with the 1859/60 Royal Commission responding to French re-armament as those defences were built further out to block access into the Solent. However the development of the motor torpedo boat - small but fast units that could threaten devastating attacks on shipping - saw Calshot re-armed. The castle itself was too small so a separate platform was built adjacent to the Henrician fortification to house six Quick Firing guns designed to counter the threat. Searchlights were fitted in the castle to illuminate targets. A boom was also rigged across the mouth of Southampton Water which was guarded by armed towers known as dolphins. Finally, in 1907, the roof of the Keep was modified to take another pair of Quick Firing guns.
Calshot took on a very different role in the twentieth century due to developments in aviation. The benefits of aircraft in support of the Fleet was not lost on the Royal Navy who established a series of air-stations at coastal locations. On 29 March 1913 Royal Navy Air Station Calshot was formally commissioned to support sea planes. Wooden hangers were built to house up to twelve aircraft with launching rails enabling them to be moved to/from the water. Naval personnel were accommodated in local cottages or at Warsash across the water. Initially used just for experimental testing, the site took on a training role at the outbreak of World War I. By 1916 though the threat to shipping in the Channel from German U-boats was such that anti-submarine patrols were flown from the base. On 1 April 1918, with the incorporation of the Royal Navy Air Service into the Royal Air Force, Calshot was transferred to the new military arm although its role predominantly remained the same. This continued through the inter-war years but, more famously, the site also hosted the 1929 and 1931 meetings of the Schneider Trophy air race. This competition encouraged development of high speed flying and was won by Great Britain on both these occasions using the Supermarine S6, an aircraft that would ultimately pave the way for the Supermarine Spitfire.
At the outbreak of World War II in 1939, the castle had been completely de-armed but a barge was moored at the site and fitted out with anti-aircraft weaponry. However as the military situation in France and the Low Countries deteriorated, resulting in the withdrawal of British forces from the continent by June 1940, the castle was fitted out with Quick Firing guns and searchlights. A supporting battery - known as the Bungalow Battery - was built on the opposite shore near Warsash in 1941 to provide additional fire-power. The two installations, along with Stone Point Battery a few miles South West of the castle, formed Calshot Fire Command. Concurrently the site also continued to function as a seaplane base – now for Short Sunderland aircraft operating in an anti-submarine capacity.
The castle was decommissioned as an airbase in the 1950s and once again became a coastguard station. But with construction of a dedicated tower for this function the old Henrician fortification became irrelevant. The castle was passed into the hands of the Ministry of Works to be preserved as a historic monument.
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Calshot Castle was a small Tudor fortification consisting of a circular Keep and curtain wall both of which were extensively modified in the late nineteenth century. The external gun battery of 1895 and the boom defences have long since been demolished but two Royal Navy seaplane hangars remain and are some of the earliest surviving examples in the country.
Calshot Castle Defences. The Henrician fort was directly adjacent to the deep water channel into Southampton Water. A battery of six Quick Firing guns was installed south of the castle in 1895 and a boom installed shortly after along with stand alone towers - known as Dolphins - to support further Quick Firing guns. The seaplane facilities were constructed from 1913 onwards. Bungalow Battery was built during WWII.
Calshot Castle Defences. Today the castle is dwarfed by the surrounding structures but originally it stood alone on the shingle spit. The sea plane hangars, originally part of the Royal Navy Air Station Calshot, can be seen to the left. The coastguard station, a role originally performed by the castle, can be seen to the right.
Keep and Curtain Wall. When first built the curtain wall was much higher with the now open gun-ports originally having been fully enclosed. The original height can be seen in the small surviving segment (left). The next tier of guns was installed in the top floor of the Keep (right). The curtain wall was lowered as part of the 1895 modifications.
Quick Firing Gun. One of a pair of Quick Firing guns installed to the roof of the Tudor Keep in order to provide a defence against Fast Motor Torpedo boats.
Supermarine S6A. Calshot hosted the Schneider Trophy air races in 1929 and 1931. On both occasions the race was won by Great Britain using the Supermarine S6 and S6B. Unfortunately neither of those aircraft have survived but the S6A, built to the same design, is on display in the Solent Sky Museum in Southampton.