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.

Traditionally the Crown had left coastal defences to the local lords and communities, playing only 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, as well as 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; 80 percent of England's exports passed through it.

[12] Milton 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.