Just outside London, where the river toys with becoming an estuary, you'll find the great container port of Tilbury. These docks are relatively modern, but the surrounding area has a venerable past. Here, the river pinches to a relatively narrow flow, making it a natural place to build fortifications. On the Kent bank lays the important river town of Gravesend. Opposite, on the Essex coast, stands Tilbury Fort, a great fortress that has protected London from naval attack since the days of Henry VIII. Today, it's a visitor attraction looked after by English Heritage, and it makes for an excellent day out.
The fort is huge, covering two and a half acres. The sturdy walls zag in and out, providing a lengthy defensive perimeter. A vast marshalling ground takes up much of the interior, peppered with artillery units from different eras. It is surrounded by several Stuart-era buildings, including the officers' barracks and a sturdily buttressed store house for gunpowder. The complex is entered through a fine neo-classical water gate and sentry house.
The fort's origins are Tudor. Henry VIII established the first, much-smaller Tilbury Fort in 1539. His daughter Elizabeth I gave her rousing Armada speech nearby in 1588. After a period of decline, Tilbury was redesigned and massively expanded in 1670, following a disastrous Dutch attack on the navy in the Medway. It saw its final action in the two World Wars. Although outdated as a fortification, it made a good mustering point for soldiers heading to the trenches, and a base for anti-aircraft guns, some of which remain on site.
While there are plenty of information boards, particularly in the magazine house, you'd be wise to pick up a free audio guide, which goes into more detail. Kids will love climbing the Teletubby-esque mounds in the north-east and south-west corners. These contain a series of part-flooded tunnels, again once used for ammo storage but now an intriguing diversion for anyone who loves exploring underground spaces.
The train from Fenchurch Street takes little more than a half hour. You then have to amble for about a mile from Tilbury Town station along featureless access roads, but it's worth the trek. Be sure to visit the World's End pub after visiting the fort, for a decent selection of ales and a small beer garden.
Tilbury Fort, operated by English Heritage, is open Wed-Sun, 10am-5pm (only weekends from early November to late March). Entrance is £4.60 (adults) and £2.80 (children).
Londonist is official media partner to Estuary, an upcoming exhibition at Museum of London Docklands.