The peninsula that Dunnottar Castle sits on is such a natural stronghold that there is evidence of use for over 5000 years. The stone building remains that can still be seen date back to the late 1200's.
The castle is a paid entry attraction, but it is worth a visit and quite amazing inside. You can also get great views of the castle from the beaches and cliff tops on either side, so you don't have to pay and it's still a very interesting walk past.