Sandown Castle was an artillery fort constructed by Henry VIII in Sandown, Kent, between 1539 and 1540. 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 0.59 acres and had 39 firing positions on the upper levels for artillery, with 31 gunloops in the basement for handguns. It cost the Crown a total of £27,092 to build the three castles of Sandown, Walmer 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, Sandown was seized by pro-Royalist insurgents and was only retaken by Parliamentary forces after several months' fighting.