Sandown Castle was one of Henry VIII's Device Forts or Henrician Castles built at Sandown, North Deal, Kent as part of Henry VIII's chain of coastal fortifications to defend England against the threat of foreign invasion. It made up a line of defences with Walmer Castle and Deal Castle to protect the strategic stretch of coast known as the Downs. These three castles were known as Castles of the Downs and were all completed in 1540. Sandown was identical to Walmer Castle with a circular keep surrounded by four large semicircular bastions for heavy artillery and protected by a stone lined dry moat. The entrance was by a drawbridge on the landward side.
Sandown Castle had fallen into ruins by the end of the 17th century and the sea had breached the moat walls by 1785. The castle was repaired and garrisoned in 1808 because of the Napoleonic Wars. In 1863 it was sold by the War Office; by 1882 the castle had been largely demolished for building materials. Further damage has been caused by coastal erosion. The foundations of the landward side of the castle survive and are in a public area with an interpretation board; they are protected as a Scheduled Monument.