Kilchurn Castle has been a ruin for 250 years, and even now, you have to be willing to work to visit its pretty remains. It's open to visitors in the Summer, and you either have to sail across to its tumbledown walls or walk to it through roads liable to flooding.
The effort is worth it, though. Built in the 1400s, it was a power base for centuries until it was abandoned and then hit by lightening in the middle of the 18th century. If you do make it inside its romantic walls, you'll see the remnants of the storm-damaged turrets, all set amidst the stunning scenery of Loch Awe in Argyll and Bute.