 St Catherine 's Castle remained in use for many years . During the English Civil War of the 1640s , it was held by the Royalist supporters of King Charles I against Parliament ; in 1684 the local burgesses reported that the fortification was in a " runious " state . The antiquarian Francis Grose visited the castle in 1786 and noted that the fortification was still being maintained at the expense of the local town . He praised its " picturesque and romantic " position but concluded that the building itself was of " little importance , either to antiquity or architecture " . At this time the castle was equipped with six cannons and it continued to be used as a battery until the end of the Napoleonic Wars in 1815 . 
