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.

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.

[a] 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.

Walmer Castle was built to defend the English coast from attack by 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.

[1] 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.

[6] 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.

[15] Walmer Castle was left to decline in the early 17th century, with little money being made available for repairs and the garrison receiving low pay, leading some members to reside in nearby Deal rather the fort itself, and to take on additional employment to supplement their wages.

[16] Walmer Castle was seized by Parliamentary forces at the start of the first English Civil War between the supporters of King Charles I and Parliament, but did not play a significant role in the remainder of the initial conflict.

[22] Parliament defeated the wider insurgency at the Battle of Maidstone at the start of June, and then sent a force under the command of Colonel Nathaniel Rich to deal with Walmer and the other castles along the Downs.

[27] The garrison at Walmer remained substantial during the period, with a governor, a corporal and 20 soldiers, but when Charles II was restored to the throne in 1660 he reduced the numbers again to a captain, lieutenant, porter and 16 men.

[28] In the Glorious Revolution of 1688 against Charles' brother, King James II, the townsfolk of Deal seized Walmer Castle on behalf of William III, the Prince of Orange.

[26] The Lord Warden was originally a medieval title linked to five key ports along the coast of England; the position's prominence had faded, but it still retained important judicial and military functions.

[38] After Pitt resigned as prime minister in 1801, fears remained of a French invasion and he formed a volunteer cavalry unit at Walmer Castle, where he lived with his new officers.

[52] James Broun-Ramsay, 1st Marquess of Dalhousie became Lord Warden; on his death, the prime minister Henry John Temple, 3rd Viscount Palmerston, took over the castle in 1861.

[32] Palmerston initially declined to buy the contents of the castle from his predecessor's estate on taking up the post, a practice which had become traditional for the Lords Warden, complaining about the high price being proposed.

He expanded the gardens, built new kennels for a hunting pack and spent many years reassembling the furniture and other objects that Pitt and Wellington had used at the castle.

[59] Smith had proposed that the historical artefacts in the castle should be protected from being removed by later Lord Wardens and suggested that government pass an Indenture of Heirlooms Bill.

[62][c] The Office's survey noted that "the lower floor ... is very inconvenient, dark and not conducive to health while the women servants have to sleep in a sort of dormitory in the slopes of the roof...

[63] The ministry acquired agreement for most of the castle to be opened to the public, with various historical objects related to the property being put on display, including some donated by King Edward VII.

[65] William Lygon, 7th Earl Beauchamp, became the Lord Warden in 1913, building a Roman Catholic chapel at the castle and holding large parties there each summer.

[67] The prime minister, H. H. Asquith, was invited by Beauchamp to use the castle during the First World War as a weekend retreat, as it had good communication links with the front line in France.

[70] Rumours spread about the parties that he had held at Walmer Castle after the war, where, according to the historian Richard Davenport-Hines, he had "behaved indiscreetly with young men".

[81] Walmer Castle retains most of its original 16th-century structure, with a tall keep, 83 feet (25 m) across, at the centre, flanked by four rounded bastions, one of which served as a gatehouse, and a moat, surrounded in turn by a curtain wall.

[85] The castle had three tiers of artillery – the heaviest and longest range weapons occupying the upper levels, including the keep – with a total of 39 firing positions, and 31 gunloops in the basement for handguns should close defence be required.

[91] The Prince Consort's and Queen Victoria's rooms in the keep are named after their use during the royal visit of 1842, although their decoration today dates from the interwar period.

[101] On the other side are the paddock, planted with Holm oaks, and a curved belt of woodland of beech, ash and chestnut trees, badly damaged in the storms of 1987 and 1990.