The curve and narrowness of the river here made it a suitable place to construct forts for the defence of London against foreign invaders. The first permanent fort at Tilbury was a D-shaped blockhouse built in 1539 by Henry VIII and initially called the "Thermitage Bulwark", because it was on the site of a hermitage dissolved in 1536. The Tilbury blockhouse was designed to cross-fire with a similar structure at New Tavern, Gravesend. During the Armada campaign, the fort was reinforced with earthworks and a palisade, and a boom of chains, ships’ masts, and cables was stretched across the Thames to Gravesend, anchored by lighters. The Fort was rebuilt under Charles I and is now owned by English Heritage.