The castle was significantly expanded and remodelled by Edward III, who turned it into a grander property, designed to defend against a potential French attack, as well as to provide the King with a convenient private residence close to London.

[4] The exact date of construction is uncertain, but it is now believed the work was conducted early in de Burgh's tenure of the site, permission being retrospectively officially confirmed through a licence to crenellate in 1230 under Henry III.

[7] The first castle built on the site was probably of an octagonal design, running along the top of the ridge, protected by square and semi-circular mural towers, with a barbican guarding the eastern entrance.

[9] The castle was built of Kentish ragstone and cemented by a mortar containing a large proportion of seashells, particularly cockleshells from the cockle beds of neighbouring Canvey Island.

[18] Originally, historians believed that Edward's decision to rebuild much of the castle was in response to the growing tensions with France; in this version of events, Hadleigh would have formed an important coastal fortification along the Thames estuary, protecting it from French raids.

[20] Whilst the entrance on the inland side remained relatively basic, the building work created a grander impression from the estuary – any visitors to London, English or French, would have passed by Queenborough Castle on the south bank and Hadleigh on the north, the combination communicating a strong sense of royal power.

[25] The English painter John Constable visited Hadleigh in 1814 and made a drawing of the castle as preparation for ten oil sketches and a single painting.

One of the sketches is currently displayed at the Tate Gallery, London, while the painting now hangs in the Yale Center for British Art at New Haven, United States.

[26] Constable's painting, "one of his most monumental works" according to the art historians Tammis Groft and Mary Mackay, depicts the early 19th-century Hadleigh Castle as a decaying, man-made structure, succumbing to the elemental power of nature.

[27] William Booth purchased Hadleigh Castle and its surrounding site in 1891 for the use of the Salvation Army, which established a farm to train the English poor prior to them being sent overseas to the British colonies.

[28] The Salvation Army gave the castle to the Ministry of Works in 1948, and it is now owned by English Heritage, classed as a scheduled monument and a Grade I listed building.