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.