The Castle 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. Comprising a keep and four circular bastions, the moated stone castle covered 0.61 acres (0.25 ha) and had 39 firing positions on the upper levels for artillery. It cost the Crown a total of £27,092 to build the three castles of Walmer, Sandown, and Deal, which lay adjacent to one another along the coast and were connected by earthwork defences. The original invasion threat passed, but during the Second English Civil War of 1648–49, Walmer was seized by pro-Royalist insurgents and was only retaken by Parliamentary forces after several months' fighting.
  In the 18th century, Walmer became the official residence of the Lord Warden of the Cinque Ports and was gradually modified from a military fortification into a private residence. Various Prime Ministers and prominent politicians were appointed as Lord Warden, including William Pitt, the Duke of Wellington and Lord Granville, who adapted parts of the Tudor castle as living spaces and constructed extensive gardens around the property. By 1904, the War Office agreed that Walmer had no remaining military utility and it passed to the Ministry of Works. Successive Lord Wardens continued to use the property but it was also opened to the public. Walmer was no longer considered a particularly comfortable or modern residence, however, and Lord Curzon blamed the poor condition of the castle for his wife's death in 1906.
  Lord Wardens since the Second World War have included Winston Churchill, Robert Menzies and Queen Elizabeth The Queen Mother, but they have made only intermittent use of Walmer Castle. In the 21st century, Walmer Castle is run as a tourist attraction by English Heritage. The interior of the castle displays a range of historical objects and pictures associated with the property and its Lord Wardens, protected since the 19th century by special legislation. The grounds include the Queen Mother's Garden, designed by Penelope Hobhouse as a 95th birthday gift for Elizabeth in 1997.

History
Walmer 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. 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.
  In 1533, Henry broke with Pope Paul III in order to annul the long-standing marriage to his wife, Catherine of Aragon and remarry. Catherine was the aunt of Charles V, the Holy Roman Emperor, and he took the annulment as a personal insult. This resulted in France and the Empire declaring an alliance against Henry in 1538, and the Pope encouraging the two countries to attack England. An invasion of England appeared certain. 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.
  Walmer and the adjacent castles of Deal and Sandown were constructed to protect the Downs in east Kent, an important anchorage formed by the Goodwin Sands which gave access to Deal Beach, on which enemy soldiers could easily be landed. The stone castles were supported by a line of four earthwork forts, known as the Great Turf, the Little Turf Bulwark, the Great White Bulwark of Clay and the Walmer Bulwark, and a 2.5-mile-long (4.0 km) defensive ditch and bank.Collectively the castles became known as the "castles of the Downs" and cost the Crown a total of £27,092 to build.
  Walmer was built between April 1539 and autumn 1540, by a team including Richard Benese as the surveyor, William Clement as the master carpenter, and Christopher Dickenson as the master mason. It was initially garrisoned by a captain, two lieutenants, two porters, ten gunners and three soldiers, at an annual cost of £174. It was probably equipped with a range of brass and cast-iron guns, along with arquebuses and bows for close defence. In 1597, a report listed the castle's artillery as comprising a cannon, a culverin, five demi-culverins, a saker, a minion and a falcon.