Duffus Castle has appeared on my blog before, it's a favourite place of ours. Although it's an ancient ruin, built in 1151, this doesn't detract from its appeal and I think it has great photographic possibilities.
A bit further away from here, on the edge of Loch Ness, is Castle Urquhart. Another in ruins but equally interesting. The best view of it is from the actual loch.....but since I haven't got one from that angle, you'll have to make do with this one! To see more monochromes click here