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 Plymothians 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.
Is there an answer to this question (If it cannot be answered, say "unanswerable"): For how many years was Plymouth under siege during the English Civil War?
four