Caerlaverock Castle is one of the strongest reasons to leave Dumfries town for the day. Its red sandstone walls form an unusual triangle, set inside a broad moat, with twin towers guarding the entrance and enough ruined structure to make the plan easy to read.
The castle was fought over, rebuilt, and finally left to ruin after the 17th century. Go for the shape as much as the history: from across the water it looks exactly like the sort of medieval stronghold people hope Scotland will produce.