Originally built by Henry VIII in 1539 to protect England against the threat of invasion from France and the Holy Roman Empire, it comprised a circular bastion, flanking wings and a keep, and in 1547 it housed 17 pieces of artillery.

Cowes 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 the local lords and communities, only taking a modest role in building and maintaining fortifications, and while France and the Empire remained in conflict with one another, maritime raids were common but an actual invasion of England seemed unlikely.

[1] Modest defences, based around simple blockhouses and towers, existed in the south-west and along the Sussex coast, augmented by a few more impressive works in the north of England, but in general the fortifications were very limited in scale.

Two castles were built on the west and east sides of the River Medina, the entrance to the principal harbour of the adjacent Isle of Wight, and were intended to attack any enemy ships that might approach.

[9] Constructed in 1539, the western fortification, which became known as West Cowes Castle, had a D-shaped bastion at the front, a round, two-storey keep at the rear and two low buildings on either side, all of which mounted artillery guns.

[13] Inspections in 1547 showed that there were 17 iron and brass artillery pieces at the fort, but that nine of these were inoperable; nonetheless it continued in use as a fortification after the original invasion scare had ended, staffed with a garrison of a captain, porter and three gunners during Queen Elizabeth's reign.

[16] An unidentified Scottish naval captain then came ashore on 16 August and captured Turney, before landing more men and taking the castle for Parliament, who held it for the rest of the war.

[20] An inspection in 1692 reported that the walls were cracked and at risk of collapse, and the antiquarian Francis Grose observed in 1785 that the castle was "strongly fenced with piles and planks" to prevent erosion from the sea.

[22] Most of the front of the keep was demolished and rebuilt with new windows, a turret for a spiral staircase was erected, new three- and two-storey residential wings were added, and a garden was created over the landward defences.

By October 1942, the castle was occupied by the British Admiralty and became the senior officer's headquarters of the HQ Naval Commander Force "J" Landing Craft base, known as HMS Vectis.