The impressive heap that is Richmond Castle, founded in 1070, has had many uses through the years, including a stint as a prison for conscientious objectors during WWI (there's a small and fascinating exhibition about their part in the castle's history – enter through the shop). The best part of a visit is the view from the top of the remarkably well-preserved 30m-high keep, which dates to the late 12th century and towers over the town.
Richmond Castle
Yorkshire Dales National Park