Erm...I knew at one time! That castle is right off of Loch Lomond. I want to say that it was once the hunting "lodge" of Robert the Bruce.

After looking around a bit. I think I'm wrong (nothing surprising). It does look like the Loch Lomond Youth Hostel (http://www.loch-lomond.net/accommodation/youthhostels/youth_hostels.html) but I can't be sure. I knew I should have taken notes about which castles were which. Oops.