Built on the point of a natural spit, Calshot Castle occupies a very strategic position on the eastern entrance to Southampton Water. The geography here is perfectly suited for the location of a gun battery, as the deep water shipping channel runs very close to the shore at this point.

Following Henry VIII's split with Rome, and his subsequent divorce from Catherine of Aragon, a very real threat of invasion was posed by a combined French and Spanish fleet. To counter this threat, Henry VIII commissioned the building of a string of coastal fortifications stretching from south Wales to the River Humber in the north east, and Calshot Castle was one of these.

Finished in 1540, Calshot Castle was designed specifically to facilitate a large amount of artillery, but also to repel an artillery attack. In fact, Henry's forts marked a significant change in English castle building. The medieval castles, although big, bold and imposing, were now an easy target for 16th century cannon. Walls and parapets of the new smaller, squat castles were curved or circular and immensely thick, enabling them to absorb and deflect incoming enemy shot. Calshot Castle is a good example of this, comprising a three storey circular keep, surrounded by a concentric curtain wall, and protected by a stone-lined moat with access via a drawbridge.

The castles defences consisted of three tiers of guns, probably culverins or demi-culverins: these were located within the curtain wall, on the second floor of the keep, and on the keep roof. The gatehouse roof also had space for an additional three guns, should defence of a landward attack be required. Much of the stone and other building material is thought to have come from the demolition of nearby abbeys, Netley and Beaulieu.

A captain, a deputy, eight gunners and five soldiers garrisoned the fort but, as is often the case in British history, the threat of invasion had largely passed by the time most of the fortifications had been built. Calshot Castle's history for the next two centuries remained largely uneventful.

In 1774, an additional floor was added to the gatehouse of the Tudor castle to provide rooms for the commander. At about the same, it is understood that the defensive wall was lowered, creating open gun embrasures from the original covered gun ports. Changes were made also to the keep roof, where the gun embrasures were filled in to provide a continuous protective parapet for riflemen.

Throughout the 19th century, Calshot Castle's fight was mainly against smugglers, rather than the massed armies of Europe. For many years, the coastguard headquarters were based at the castle. However, with the advancement of the torpedo and the torpedo boat, the castle's strategic position was again realised, and it was handed back to the War Office in 1894 for modernisation and re-armament.

Found to be too small for the chosen number of modern quick firing (QF) guns required, a separate battery was built to the south of the Tudor castle, comprising 2 x 4.7 inch (118mm) and 4 x 12 pound QF guns. Within the castle itself, the seaward facing curtain wall embrasures were converted to facilitate three searchlights. Generators, located in the keep basement and powered by oil engines, provided the required electrical current. Modifications were also made to the gatehouse to provide a guardroom and detention cell.

The last major structural change to Calshot Castle was undertaken in 1907, when the keep roof was strengthened to accommodate 2 x 12 pound QF guns. The generator equipment was removed, and the keep basement was turned into a magazine, with a hoist through the centre linked to the roof.

Dramatic changes to the surrounding area occurred between the World Wars, as it first became the Calshot Royal Naval Air Station, and then the Royal Air Force School for Naval Co-operation and Aerial Navigation. During the Second World War, QF guns were re-mounted on the castle roof and additional searchlights were installed. Calshot RAF Air Station had become a centre for the maintenance and repair of seaplanes.

When the RAF Station closed in 1953, Calshot Castle reverted back to providing coastguard accommodation, until their adjacent new tower was completed. At that time, English Heritage took over the guardianship of the castle, restoring it to its former appearance pre-1914.