It formed part of the King's Device programme to protect against invasion from France and the Holy Roman Empire, and defended the Weymouth Bay anchorage.

Sandsfoot 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.

[3] 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.

[8] 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.

[10] Sandsfoot was a blockhouse, intended to defeat enemy ships using a battery of heavy artillery, and had minimal protection against an attack from the land.

[12][b] There was probably an early agreement that the nearby village of Wyke Regis had a responsibility to support the castle, and in exchange they came to traditionally enjoy an exemption from taxes and militia duties.

[19][b] A survey in 1623 carried out by Sir Richard Morryson showed the castle to be equipped with ten iron guns – one culverin, five demi-culverins, two sakers, a minion and a falcon – and garrisoned by its captain, five gunners and three soldiers.

[21][b] During the English Civil War between the supporters of Charles I and Parliament, Weymouth was predominantly Parliamentarian in loyalty and the surrounding forts were held by their garrisons.

[22] Robert Dormer, the Earl of Carnarvon, entered Dorset with an army in 1643 and Weymouth surrendered, resulting in Sandsfoot Castle being controlled by the Royalists between August 1643 and June 1644.

[24][c] Robert Devereux, the Earl of Essex, then retook the county for Parliament; Colonel William Ashburnham, the Royalist governor of Weymouth, retreated to Portland Castle without a fight.

[29] An argument then broke out between Humphrey Weld, the lieutenant-governor of Portland and captain of Sandsfoot Castle, and Charles Stewart, the Duke of Richmond, over the control of the local defences.

[30] The village of Wyke Regis petitioned Weld in a bid to prevent the demobilisation, concerned that their traditional exemptions from militia service would be revoked by the Duke.

[38] The ruined castle was drawn and painted by various artists in the 18th and 19th centuries, including Samuel Buck, J. H. Grimm, C. Sawyer and Edward Pritchard.

[43] As part of the work, a three-dimensional laser scan of the stonework was undertaken, and a steel and oak walkway was installed around the interior of the castle.