Castle Urquhart is perched on the edge of Loch Ness and it's the ruins you see in all of the photos of Loch Ness. It enters the records around 580 A.D., so they think it's been around that long in one form or another. Being situated strategically on the loch, it's changed hands many times throughout history, both between the clans and the English. It seems that the clans Durward, Mac Donald and Grant have all been in charge of it throughout it's history. In 1692 the English soldiers blew it up as they left it for the last time.
I think it's especially compelling at night, so that would be the photos I have of it!