 During the English Civil War Plymouth sided with the Parliamentarians and was besieged for almost four years by the Royalists . The last major attack by the Royalist was by Sir Richard Grenville leading thousands of soldiers towards Plymouth , but they were defeated by the <unk> at Freedom Fields Park . The civil war ended as a Parliamentary win , but monarchy was restored by King Charles II in 1660 , who imprisoned many of the Parliamentary heroes on Drake 's Island . Construction of the Royal Citadel began in 1665 , after the Restoration ; it was armed with cannon facing both out to sea and into the town , rumoured to be a reminder to residents not to oppose the Crown . Mount Batten tower also dates from around this time . 
