The Royal Citadel in Plymouth is a 17th century fortress. It was built in 1665 as a defence against a threatened Dutch invasion, and was England’s most important defence for over 100 years. Perhaps because of Plymouth’s Parliamentary allegiance during the English Civil War, the Citadel’s canons cannot only fire out to sea, but also on the rebellious city itself. Regularly strengthened throughout its lifetime, the Citadel continues to be used by the military today, however visitors may still enjoy scheduled guided tours during the summer months.