Gravesend Blockhouse was built by Henry VIII on the Thames Estuary in 1539 as part of his chain of coastal defences in response to the threat of invasion. It was one of five blockhouses built along this stretch of the river Thames to defend the approach to London and the dockyards at Woolwich and Deptford; the others being at Tilbury, Higham, Milton and East Tilbury.
In the 1780s the eastern gun lines were remodelled and the blockhouse was converted into a magazine. In 1834 the gun lines were levelled and the blockhouse was demolished in 1844.
The appearance of the blockhouse is known from a plan by John Romer made in 1715. This shows that the small D-shaped blockhouse had a curved front facing the river, two angled faces on the landward side and a curved bastion on the west side.
Some of the ruins of the blockhouse have survived. The western two-thirds of the semi-circular front wall with its gun-ports has been consolidated and is on display. The remainder of the site is under a car park to the east and a road to the south.