The castle seen here is a matte painting while the background of the scenes looking over Loch Katrine and towards Ben Venue is real.
While the interiors of the castle are the real Blackness Castle near Edinburgh the castle walls seen here have little resemblance to it, although the central tower or keep is visibly based on the one in Blackness Castle.
The still below is of Loch Katrine with a view towards Ben A'an, the pointy mountain seen in the distance to the left of the hanging corpse.
The scenes shown appear to be filmed at the point on the north shore of Loch Katrine approximately 3km (2 miles) from Trossachs Pier which has a large car park, visitor centre, restaurant and bicycle hire shop. And of course boat rides on the fabulous steamer SS Sir Walter Scott.
Somes sources call this point Brenachoile Point whereas it is shown as Ruinn Dubh Aird on the Ordnance Survey map of the area. There is a road going along past this location but it is walking or bicycles only.