Royal Citadel

The Royal Citadel sits on a headland overlooking Plymouth Sound, a proper military fortress that's been standing watch since 1666. Walking up to it from the Hoe is straightforward enough, though you'll feel the climb in your legs - the views back across the water make it worthwhile. Inside, it's still an active barracks, which gives the place an authentic working atmosphere rather than that sterile museum feel. You can wander the ramparts, poke around the gun emplacements, and actually picture what it was built to defend against. The visitor facilities are decent without being fancy, and the guides know their stuff about the Civil War period and the centuries that followed.

Histor types will spend a good two hours here poking into every corner. Families with restless kids might find it drags a bit - there's not much interactive stuff, and if your children aren't keen on old fortifications, they'll be ready to leave after forty minutes. The walk itself is pleasant, and you're right on the edge of Plymouth itself, so you can easily combine it with the Hoe or grab lunch in town afterwards.

If you're based at Dartmoor Domes just up the road, the Citadel makes for a solid morning or afternoon outing - close enough not to eat up half your day travelling. Check opening times before you go, as it's still operational military property and access can be restricted. Worth the trip if you're into heritage sites with actual character rather than something too polished and corporate.