Upnor Castle belongs to the genre of Henrician cosastal forts but is an Elizabethan addition to the chain. It was begun in 1599 to guard the approach to the new dockyard at Chatham, lying two miles away near the estuary of the River Medway. Sir Richard Lee interrupted his work on the fortifications at Berwick-on-Tweed, to come and design this fort, but construction dragged on for eight years. In 1599-1601,
Upnor was enlarged but it had to wait until 1667 to face enemy action. In that year, the Dutch, under Admiral de Ruyter, sailed into the Medway and set fire to much of the English fleet. The castle was unable to offer any effective resistance and in the following year a new chain of defenses was begun, Upnor being relegated to the role of storehouse and magazine. Military occupation of one kind or another continued until the Second World War.
As originally conceived, the castle comprised an oblong blockhouse, set in the middle of a curious screen wall terminating at each end in a stair turret. This building provided accommodation for the garrison, defense being concentrated upon the low, pointed bastion facing the Medway.
Pointed bastions were devised as a defense against artillery in Renaissance Italy. Sir Richard Lee built several along his new ramparts at Berwick, but the Upnor bastion does not have the characteristic “arrowhead” plan resulting from a narrow collar. Its riverside setting made that unnecessary. However, since only one side of the bastion faces upriver, there were insufficient gun emplacements to fire effectively on an approaching fleet-this was the problem in 1667.
The late Elizabethan enlargement provided defenses on the landward side. A walled courtyard was created in front of the blockhouse, with towers where the new curtain joins the screen wall. The courtyard is entered through a gate tower retaining the traditional obstacle of a drawbridge.