Dunnottar Castle, near Stonehaven, is an extremely impressive sight standing on massive coastal rock with precipitous cliffs.
This has been a defensive site since at least the 7th century and probably longer. However, the oldest portion of what now stands probably dates back to the 1400s with many of the buildings dating to around 1600.
St Ninian retreated here and built a chapel in Pictish times. William Wallace also come here (and burned St Ninian's chapel and the wooden castle that stood at the time). The Scottish Crown Jewels were held here whilst Oliver Cromwell laid seige for eight months; when the heavy artillery arrived the regalia were lowered over one of the seaward cliffs and sneaked away. Damage from the seige is still very visible.
Mary Queen of Scots also stayed here - of course.