It formed part of the King's Device programme to protect against invasion from France and the Holy Roman Empire, and defended the strategically important Downs anchorage off the English coast.

Comprising a keep and four circular bastions, the moated stone castle covered 0.59 acres (0.24 ha) and had 39 firing positions on the upper levels for artillery, with 31 gunloops in the basement for handguns.

[a] The original invasion threat passed, but during the Second English Civil War of 1648–49, Sandown was seized by pro-Royalist insurgents and was only retaken by Parliamentary forces after several months' fighting.

Sandown 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.

[12] It was nearly identical to its sister castle at Walmer, with a tall keep, 83 feet (25 m) across, at the centre, and flanked by four rounded bastions, also called lunettes, and a moat, which was surrounded in turn by a curtain wall.

[16] Sandown had three tiers of artillery – the heaviest and longest range weapons occupying the upper levels – with a total of 39 firing positions, and 31 gunloops in the basement for handguns should close defence be required.

[21][c] Sandown Castle did not play a prominent role during the first English Civil War that broke out between the supporters of King Charles I and Parliament; it was taken by Parliamentary forces at the start of the conflict in 1642.

[28] Parliament defeated the wider insurgency at the Battle of Maidstone at the start of June, and then sent a force under the command of Colonel Nathaniel Rich to deal with Sandown and the other castles along the Downs.

[34] In the Glorious Revolution of 1688 against Charles' brother, King James II, the townsfolk of Deal seized Sandown Castle on behalf of William III, the Prince of Orange.

[35] The former Parliamentarian, Colonel John Hutchinson, was infamously detained by the Crown at Sandown in 1664; his wife, Lucy, who subsequently widely publicised his case, described the castle as ruinous and his imprisonment there as inhumane.

[43] In 1883 architectural historian Hayter Lewis complained to the British Archaeological Association that more needed to be done to protect the ruins, contrasting their poor treatment with that of the other castles along the coast.