Walmer 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 one of three forts constructed to defend the Downs, an area of safe anchorage protected by the Goodwin Sands. The other forts were at Deal and Sandown.

At the centre of Walmer Castle is a circular keep, surrounded by an open courtyard and protected by a concentric wall, from which four, squat, semi-circular bastions project. The northern bastion forms the gatehouse and would have had a gun on its roof; the other bastions would have had guns mounted inside them and on the roof. The central keep would also have had guns mounted on its roof giving the castle the capacity to mount 39 guns. A gallery running around the castle at basement level has 32 loops for hand-guns covering the moat.

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, with Walmer surrendering first after a three week siege.

In 1708 Walmer Castle took on a new role as the residence of the Lords Warden of the Cinque Ports. The Cinque Ports Confederation originated in the 11th century when the five ports of Hastings, Romney, Hythe, Dover and Sandwich joined forces to provide ships and men for the defence of the coast and protection of cross-channel trade. In return for these services they received substantial local privileges including immunity from all external courts of justice and from national taxation. In the 13th century the office of Warden was instituted to oversee and regulate the affairs of the Confederation. Initially this position carried real power, but with the forming of a Royal Navy and the decline of the Cinque Ports, the role of Warden became more of an honorary position bestowed to those who had given distinguished service to the state.

Over the years successive Wardens converted the fort and its grounds into a comfortable country house and gardens. Resident Wardens included William Pitt the Younger, the Duke of Wellington, Sir Winston Churchill and Her Majesty Queen Elizabeth the Queen Mother. Memorabilia from these past Wardens, including two rooms dedicated to the Duke of Wellington, can be viewed at the castle.