 Milton 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 . It was built at Milton , near Gravesend in Kent at a strategic point along the River Thames , and was operational by 1540 . Equipped with 30 pieces of artillery and a garrison of 12 men and a captain , it was probably a two @-@ storey , D @-@ shaped building , designed to prevent enemy ships from progressing further up the river or landing an invasion force . It was stripped of its artillery in 1553 and was demolished between 1557 and 1558 ; nothing remains of the building above ground , although archaeological investigations in the 1970s uncovered parts of the blockhouse 's foundations . 
