The Royal Citadel is a star-shaped 17th-century fortress that sits on the headland just above the Barbican, commanding the entrance to Plymouth Sound. It was built for Charles II between 1665 and 1675, replacing an earlier Tudor earthwork, and remains an active military base — home to 29 Commando Regiment Royal Artillery — which gives it a maintained, working quality that many historic forts lack.
Guided tours run in summer (May to September, most days), taking visitors inside the walls to see the baroque gateway, the garrison chapel, the parade ground, and the ramparts with their views over the Sound. The tours are worthwhile — the interior is not accessible otherwise, and the guide brings the politics of the Restoration and the Civil War's aftermath to life. Check the English Heritage website or the Tourism office for tour times and booking.