The small castle on an island in Loch an Eilein on the Rothiemurchus Estate near Aviemore appeared in episode 4 of the first series of Monarch of the Glen. The castle was one of the homes of the infamous Alexander Stewart, 1st Earl of Buchan also known as the Wolf of Badenoch. Thanks to Ruth French for identifying this location.
From Aviemore take the B9152 south for a mile or so, turn left onto the B970 and then left after a mile onto a minor road signposted to the loch. After another mile or so you will come to a large paying car park where the road ends. During the summer months there is a visitor centre.