 Upnor Castle fell into Parliamentary hands without a fight when the English Civil War broke out in 1642, and was subsequently used to intern Royalist officers.In May 1648, a Royalist uprising took place in Kent and Essex, with the royalists seizing a number of towns, including Gravesend, Rochester, Dover, and Maidstone.The Royalists were defeated in the Battle of Maidstone on 1 June, and the castle was restored to Parliamentary hands.