Deal Castle was built by Henry VIII in 1539-40 as an artillery fortress to counter the threat of invasion from Catholic France and Spain. It was the largest of three forts constructed to defend the area of safe anchorage known as the Downs. The other forts were at Walmer, now the official residence of the Lords Warden of the Cinque Ports, and Sandown, which was largely demolished in the middle of the 19th century.

At the centre of Deal Castle is a round tower, strongly constructed to carry guns on its roof. Around its base are six small semi-circular bastions that overlook the outer wall. This has six more massive rounded bastions, one of which forms the gatehouse. These outer bastions originally had space for four guns on their flat roofs and a further three guns in rooms below. The design of the castle meant that a total of 66 guns could be mounted, and a further 53 handguns could be fired through firing-loops at basement level. Around 1570 the six outer bastions were filled with earth, probably to strengthen the gun mounts on the roof.

The defences were never put to the test during the Tudor period and it wasn't until 1648, during the Civil War, that the castle finally came under siege. The three 'castles of the Downs' were initially held for Parliament, but the forces switched allegiance to support the Royalist cause. It took Parliamentary forces, led by Colonel Rich, nearly three months to defeat the three castles, during which time they suffered great damage. Repairs were made to the castles and in the late 1720's more alterations were made to Deal Castle to provide more comfortable accommodation. The castle ceased to have a defensive role by the end of the Napoleonic wars in 1815, but it remained the home of the Captain of the Castle until the Second World War.