Upnor Castle is a well preserved gun fort, built between 1559 and 1567 on the orders of Queen Elizabeth I to defend her warships at anchor in the reaches of the Medway and Chatham Dockyard. The castle is fronted by a water bastion jutting out into the River Medway.

The fort was not well maintained and proved ineffective when the Dutch, under the command of Admiral de Ruyter, sailed up the Medway in June 1667 to attack the dockyard. The Dutch met very little resistance, and when they left two days later they had destroyed and captured a large number of the Royal Navy ships anchored at Chatham. The attack caused a reappraisal of the defences of the Medway estuary, and in 1668 Upnor was downgraded to be used as a magazine, supplying powder and shot to warships anchored in the river.