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.

It cost the Crown a total of £27,092 to build the three castles of Deal, Sandown and Walmer, which lay adjacent to one another along the coast and were connected by earthwork defences.

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

Although it remained armed, Deal was adapted by Sir John Norris and Lord Carrington during the 18th and 19th centuries to form a more suitable private house for the castle's captain, which was by now an honorary position.

Early in the Second World War, the captain's quarters were destroyed by German bombing, forcing Deal's then-captain, William Birdwood, to move to Hampton Court Palace and the castle became an observation post for an artillery battery placed along the shore line.

Deal 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 limited 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.

[2] 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.

[7] 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.

[12][a] It is not known who designed Deal Castle, but Sir Edward Ryngeley and Thomas Wingfield served as the commissioners for the project, with Robert Loyrde and David Marten as the paymaster and comptroller, and Christopher Dickenson and William Clement as the master mason and carpenters.

[24] By 1615, Deal Castle was in a poor state, its outer walls damaged by storms and coastal erosion, while a survey suggested repairs estimated at £396 were necessary.

[27] As its condition deteriorated, fighting between Dutch, French and Spanish ships in the Downs became common, increasing the strategic importance of fortifications along the Kent coast.

[28] A new survey in 1624 estimated the cost of repairs had escalated to £1,243 and George Villiers, 1st Duke of Buckingham, suggested nearby Camber Castle be demolished to provide materiel for the work.

[32] When the Second English Civil War broke out in 1648, the Parliamentarian navy was based in the Downs, protected by Walmer and the other Henrician castles, but by May a Royalist insurrection was under way across Kent.

[44] Rich became the new Captain and reported the castle had been badly damaged during the conflict, being "much torn and spoiled with the granades", and he estimated repairs work would cost at least £500.

[46] In light of the Dutch threat, Deal Castle was maintained and kept equipped with powder, and was reinforced with earthworks and soldiers at the start of the First Anglo-Dutch War in 1652.

[49] Following the December 1688 Glorious Revolution which removed James II, the townsfolk of Deal seized the castle in support of William of Orange, and took steps to defend the Downs against a feared Irish invasion which never materialised.

[56] According to Samuel Wilberforce, Carrington had hoped to charge the cost of the work to the Treasury, but when he attempted to submit the claims, they were rejected by Pitt and he had to pay for the improvements himself.

[61] During the Second World War, in November 1940, during the Blitz, German bombers destroyed much of the captain's quarters, forcing the incumbent, William Birdwood, to move to Hampton Court Palace.

[72] The castle originally had four tiers of artillery – the heaviest and longest-range weapons occupying the upper levels, including the keep – with a total of 66 firing positions and another 53 gunloops in the basement for handguns, should close defence be required.