Upnor Castle is a compact Tudor gun fort on the River Medway, built to defend the royal dockyard and the warships at Chatham. Its brick walls, water gate, and riverside position make the military purpose immediately clear: this was a castle designed for cannon, not romantic feasting halls.
The irony is that Dutch ships still broke through in 1667 during the Raid on the Medway, one of the more humiliating moments in English naval history. That story gives the visit its bite, and the river setting makes it an easy pairing with Rochester and Chatham's dockyard heritage.