Sandown Castle, a Henrician artillery castle, was built in 1539-40 by Henry VIII as part of his chain of coastal defences in response to the growing threat of invasion at that time. The castle, which was designed to resemble a Tudor rose, was built, along with Deal and Walmer Castles, to protect the good landing grounds and strategic anchorage between the Goodwin Sands and the coast, an area known as the Downs. A series of bulwarks, or earthen defences, were also built along the coast between the three castles.
The castle was in a ruinous state by the late 17th century and the sea had breached the moat walls by 1785. The castle was repaired in 1808 and it was garrisoned in the Napoleonic Wars. However, in 1863 it was sold by the War Office as building material and by 1882 the castle had been largely demolished.
Although most of Sandown Castle has been destroyed there exist a number of early plans and views which provide information on how its original form. It was identical to Walmer Castle with a circular keep surrounded by four bastions separated by an inner moat and surrounded by an external moat. The entrance was by a drawbridge on the landward side. The outer bastions were massive structures each with a gun room at ground level and a gun-platform on the roof to mount artillery on. The moat was protected by a fourth tier of hand-guns which were situated on a gallery which ran around the whole castle at basement level. There were 39 gun-ports for heavy armaments and 39 hand-gun loops to control the moat.
Today all that remains of Sandown Castle is part of the west side of the castle including bastions and a section of the central tower. Remains of the castle have been incorporated into the sea defences.