Walmer Castle was built in 1539–1540 on Henry VIII's orders after a European alliance created fear of invasion. It formed part of the king's Device program of coastal forts and worked with nearby Deal and Sandown castles to protect the Downs, an important anchorage off Kent. A central circular keep, four rounded bastions, low walls, gun ports, magazine space, and a surrounding ditch created a compact artillery platform intended to resist cannon fire and command the beach.

The threatened invasion did not occur, but captains, gunners, soldiers, and ordnance officials maintained Walmer as defenses and weapons changed. Its low concentric design was well suited to Tudor guns but offered limited domestic comfort. During the English Civil Wars, local Royalists seized the castle in 1648 as part of a Kentish rising. Parliamentarian troops besieged and recovered Walmer, demonstrating that the old fort still controlled useful coastal ground even though its original strategic system was already being overtaken by newer military methods.

From the eighteenth century, Walmer became the official residence of the Lord Warden of the Cinque Ports. The office had once carried substantial authority over maritime defense, trade, justice, and the historic ports of southeastern England, but its duties gradually became ceremonial. Residential wings, larger windows, furnishings, and service rooms transformed the austere gun fort into a seaside house. Gardens, woodland, walks, and kitchen grounds extended domestic life beyond the ditch while preserving the Tudor plan.

Several prominent officeholders shaped the castle. William Pitt the Younger used Walmer while organizing coastal defense during war with Revolutionary and Napoleonic France. Arthur Wellesley, duke of Wellington, served as Lord Warden from 1829 and regularly stayed there. He died at Walmer in 1852, and his bedroom, campaign furniture, personal objects, and funeral associations became central to the castle's public memory. Later wardens included political figures and members of the royal family, each adapting the residence to contemporary expectations.

Walmer remains both a historic monument and the official residence attached to the wardenship, though the office no longer commands ships or courts. English Heritage conserves the Tudor masonry, later interiors, collections, and extensive gardens as interdependent layers. The site makes a striking architectural transition visible: rounded bastions built to repel an invasion became drawing rooms and garden terraces for statesmen. Its importance lies not only in Henry VIII or Wellington, but in three centuries during which a national defense installation became an official country residence without losing the form of its original artillery fort.