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 France and the Holy Roman Empire. It was built at Gravesend in Kent along a strategic point along the River Thames and was operational by 1540. A two-storey, D-shaped building built from brick and stone, it had a circular bastion overlooking the river and gun platforms extending out to the east and west.