 Henry issued an order , called a " device " , in 1539 , giving instructions for the " defence of the realm in time of invasion " and the construction of forts along the English coastline . Under this programme of work the River Thames was protected by a mutually reinforcing network of blockhouses at Gravesend , Milton , and Higham on the south side of the river , and Tilbury and East Tilbury on the opposite bank . 
