The castle saw action again during the English Civil War, when it was in the front line between Royalist Monmouthshire and Parliamentarian Gloucestershire. It was held by the Royalists and besieged in both 1645 and in 1648, eventually falling to the Parliamentarian forces on 25 May 1648. A memorial to Sir Nicholas Kemeys, who led the Royalist defence during the Second Civil war and was killed in combat after refusing to surrender after the castle's fall, lies within the keep.