Sandgate Castle was originally built as an artillery castle in 1539-1540 by Henry VIII, as part of his chain of coastal defences in response to the threat of invasion. It was built to defend a vulnerable stretch of coastline and due to its proximity to the French coast the site has been constantly defended and refortified. In 1715-1716 the keep was re-roofed and the seaward battery rebuilt following damage by the spring tides. In 1805-6, during the Napoleonic wars, a major series of alterations were carried out on the castle to convert it into a gun-fort or tower. The tops of the original defensive towers were removed and the central tower converted into a Martello style tower mounting a coastal battery. In the late 1850s a new magazine was built and alterations made to the existing gun emplacements. Pillboxes were constructed at the castle during the Second World War and in the 1950s most of the outer wall on the south side was destroyed by coastal erosion. By 1893, the castle had become a private house and was restored in 1975 under the supervision of the Department of Environment.
Little remains of the original Henrician castle due to the subsequent remodelling that took place but elements are incorporated into the later fortifications. It originally comprised of a large three-storey central tower or keep which was surrounded by two concentric curtain walls. The inner curtain wall had three round towers and the outer curtain wall had a three-storey gatehouse to the north and a rectangular building or "barbican" connecting it to the central tower. All these buildings were originally roofed, and the castle was designed so that it rose progressively from the outside in to provide three or four tiers of heavy guns. These were positioned behind 65 embrasures or gun-ports and there were also gun-loops in the lower levels of the towers and buildings to provide flanking fire.