It formed part of the King's Device programme to protect against invasion from France and the Holy Roman Empire, and it defended Southampton Water near the Solent.

Netley 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, with a few more impressive works in the north of England, but in general the fortifications were very limited in scale.

[6] 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.

[8] William Paulet, later the Marquess of Winchester, was in charge of the construction, which took place in either 1542 or 1544 and Henry granted him various estates to pay for the support of the fort and its garrison.

[9] The castle took the form of a central keep, 19.5 by 14 metres (64 by 46 ft) across with deep embrasures along the battlements, and a gun platform on either side; its design resembled that at nearby Southsea.

[12] A description of the castle around this time suggested it stood "in the midst of a thicket of trees, on a little hill close to the beach" and formed "a striking object seen from the water".

[8] It proved expensive to maintain and the surrounding land was gradually sold off, until the Southampton University Hospital NHS Trust finally decided to close it in 1998.