Fascinating. A sea stack off the Aberdeenshire coast has been confirmed as the site of an early Pictish fort in the 3rd and 4th century AD. It is a small fort – but then again a sea stack is not normally very expansive. It was dwarfed by a near neighbour – but later (5th and 6th centuries AD), Dunneter Castle. The big mystery is how the Pictish warriors regularly got to scale the sea stack – which is narrow, very tall, and doesn't come with a footpath to the top. Nowadays the sea stack can only be accessed by using ropes at low tide.
It is thought the sea stack must have been bigger and the means of gaining the top has been lost by erosion over the last couple of thousand years. However, the site was short lived – it quickly went out of use. Why?
One answer may be that sea levels around that part of the Scottish coast have fluctuated. In the 3rd century the Picts became a problem for the Romans and it has been suggested that rising seas (and expanding lochs) caused them to try and migrate into what is now England as well as the Scottish lowlands. If the sea levels had risen dramatically, if only for a short time, the sea stack may not have been such a formidable place to situate a fort, and when the waters subsided they would have been forced to abandon the sea stack and move elsewhere, such as Dunneter. See http://phys.org/print357294480.html