The Eilean Donan Castle, in Dornie... you can get access as it's open to the public (or at least it was last time I went). It's also illuminated at night, and they used to light under the arches too, but I don't think they do anymore.
Fantastic scenery in the area, we stayed at a B&B on the shore of that loch... the castle was the view out of our bedroom window!