It formed part of the King's Device programme to protect England against invasion from France and the Holy Roman Empire, and defended a vulnerable point along the coast.

The height of the castle was significantly reduced and the keep was turned into a Martello tower; when the work was completed, it was armed with ten 24-pounder (11 kg) guns and could hold a garrison of 40 men.

Sandgate 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.

[7] Sandgate was intended to defend a vulnerable point along the Kent cliffs, just west of Folkestone, where an enemy force could potentially land and make their way easily further inland.

[8] Sandgate's construction was supervised by the Moravian engineer Stefan von Haschenperg, and Thomas Cockys and Richard Keys acted as commissioners for the project.

[23] Elizabeth I visited the fortification in 1573, and also used it to imprison the courtier Thomas Keyes for a period, after he married Lady Mary Grey against the Queen's wishes.

[29][a] Four years later, amid fears of war with France and Spain, the castle's captain, Richard Chalcroft, reported that the fortification was in such a poor condition that "neither habitable or defensible against any assault, nor any way fit to command the roads".

[30] An inspection team observed that it was straightforward to climb over the castle's ruined walls and rotten timbers, and that as a result its artillery had been dismounted and placed along the beach instead.

[31] Sandgate Castle was seized in 1642 by Parliamentary forces at the start of the first English Civil War between the supporters of King Charles I and Parliament, although its captain, Richard Hippesley, remained in post.

The Parliamentary navy was based in Kent, protected by the other Henrician castles of Walmer, Deal and Sandown, but by May a Royalist insurrection was under way across the county, and the fleet joined the rebellion.

[34] Parliament defeated the wider insurgency at the Battle of Maidstone at the start of June, however, and then sent a force under the command of Colonel Rich to deal with the Kentish castles.

[35] Sandgate was still occupied by the Royalists that August, when Rich sent forces to prevent its garrison intervening to disrupt his assault on Deal and Sandown, but was recaptured soon after.

[36] During the interregnum, Hippesley initially continued as captain of Sandgate, until he was replaced in 1653, resulting in complaints from him that he had been unfairly treated and that he was owed money by Parliament.

[26] When Charles II was restored to the throne in 1660, Sandgate and the other Device Forts initially remained at the heart of the south coast defences, but by now their design was antiquated.

[46] The ground floor of the keep included a brick-built magazine, and the roof, supported by a central pillar running up through the building, held a single, large gun emplacement.

[59] In 1975, Peter and Barbara McGregor began to restore the ruins of the castle, with the support of the Department of the Environment, Kent County Council and the British Army.