What river is the castle that a stone was taken from to build the L-plan tower for Barscobe Castle on?
Barscobe Castle:  The L-plan tower was constructed using stone taken from Threave Castle. Threave Castle: Threave Castle is situated on an island in the River Dee, 2.5 km west of Castle Douglas in Dumfries and Galloway, south-west Scotland.
River Dee