Little is left today of the 14th Century castle at Threave. It is possible for tourists to visit this castle only during the summer months of June - August. Even then, it is only accessible via a small boat and all that remains is the old tower. The castle is North of A75 highway, and 3 miles West of the town Castle Douglas. It is sign-posted from the main highway, but you do have to leave your car in a farmyard and walk the rest of the way down to the reeds of the River Dee on an occasionally muddy path. There you ring a bell and a boatman will come to ferry you across to the marshy island on the river and the tower that remains.
George Carruthers, 6th Laird and 2nd Baron Holmains, was at one time Captain of Threave Castle. There was a close connection between George and Lord Maxwell, the Earl of Morton. George and his son Charles furnished a troop of cavalry in 1585 for assisting the Earl in presenting a strong force for the Borders. Charles served as a Cornet in that mounted garrison.
For their support of Maxwell in 1585, they obtained a remission by Act of Parliament under what was known as the Pacification Act of Perth. And, when Lord Maxwell and Sir James Johnstone of that Ilk entered a bond of mutual amity, George Carruthers of Holmains was one of the attesting witnesses. All through the feud between the Maxwells and the Johnstones, the Holmains part of the family played a considerable part in support of the Maxwells - though there is no evidence that they supported the Maxwells at the Battle of Dryfe Sands.
Threave, along with Lochmaben Castle and Caerlaverock Castle, were the main defenses along the Scotland England border and, therefore, historical points in the ongoing Border Wars. Threave Castle, not to be confused with the mansion of Threave Gardens which is a popular tourist attraction, was an early home of the Black Douglases, who were the Earls of Nithsdale and the Lords of Galloway. The castle was dismantled during the religious wars of the mid-17th century and was not restored.
Further history of the castle can be obtained from this link.