Gravesend Blockhouse was an artillery fortification constructed as part of Henry VIII's Device plan of 1539, in response to fears of an imminent invasion of England by European countries.

Gravesend Blockhouse was built as a consequence of international tensions between England, France and the Holy Roman Empire in the final years of the reign of King Henry VIII.

Traditionally the Crown had left coastal defences to the local lords and communities, only taking a modest role in building and maintaining fortifications, and while France and the Empire remained in conflict with one another, maritime raids were common but an actual invasion of England seemed unlikely.

[1] Modest defences, based around simple blockhouses and towers, existed in the south-west and along the Sussex coast, with a few more impressive works in the north of England, but in general the fortifications were very limited in scale.

London and the newly constructed royal dockyards of Deptford and Woolwich were vulnerable to seaborne attacks arriving up the Thames estuary, which was then a major maritime route, with 80 percent of England's exports passing through it.

[12] Gravesend Blockhouse was designed by the Clerk of the King's Works, James Nedeham, and the Master of Ordnance, Christopher Morice, with Robert Lorde serving as the paymaster for the project and Lionel Martin, John Ganyn and Mr Travers acting as the local overseers.

[21] In 1553, orders were issued for the artillery pieces to be removed from Gravesend Blockhouse and taken to the Tower of London, although the historian Victor Smith casts doubt on whether this was actually carried out.

[23] Fears of an invasion persisted for many years afterwards and in 1598 Charles Howard, the Lord High Admiral, expressed his concerns about the effectiveness of the Gravesend Blockhouse in protecting the Thames.

[23] A 1600 survey showed 10 pieces of artillery to be ineffective, while the gun platforms on either side of the fort were in bad condition and 2,828 feet (862 m) of planking, 650 joists and over 19 cartloads of other timber was needed for the repairs.

[32] Shortly after the Dutch raids, Sir John was removed from his post for apparently demanding payments from ships passing by the blockhouse, a complaint which was repeated in later years under subsequent captains.

[32] By the start of the 18th century a complex of building had grown up around the original blockhouse, which now had a pier, a dock and two wharfs alongside it, and a large house built by the King's brother, James the Duke of York, after his return to England, as well as the two lines of approximately 20 guns stretching on either side along the river; it had a garrison of a sergeant, 20 soldiers and a gunner on loan from Tilbury Fort.

[35] Amid rising concerns over the threat of a French invasion, Sir Thomas Page surveyed the blockhouse in 1778 and concluded that its guns were too closely packed together and that they could not easily fire down-river, proposing that a larger fort be built along the Thames to the east to rectify this problem.