The evocative ruins of Dunnottar Castle occupy a large, rocky, headland jutting into the North Sea, accessed by a narrow strip from the mainland. Though the current ruins date largely from the 15th and 16th centuries, its history goes back to the early medieval period, at least. Dunnottar was attacked by the Vikings, captured from the English by William Wallace, was famously where the Honours of Scotland (the Scottish Crown Jewels) were hidden from Oliver Cromwell's forces, and played its part in the Jacobite Rebellions of the 18th century.
NOTE: The castle can be closed to visitors in bad weather - essential that you check their website before visiting. There is also limited car parking - the castle is not in Stonehaven itself - it's about a 20 minute walk along the coast.