This was the seat of the infamous Black Douglases and is located on an islet in the Southern River Dee, which flows into the Irish Sea at Kirkcudbright.
The powerful Black Douglases made and unmade Kings, until James II had had enough of them and hauled the giant cannon "Mons Meg" from Edinburgh and through the marshes to reach the castle, the cannon when fired created a large hole in the castle wall which can still be seen.
The Black Douglases did not tangle with the Crown again after 1455.
You need to walk some little distance from the car park (good exercise) and then be ferried (rowing boat) across to the islet on which the castle is situated.
Then I suggest you retrace your steps and visit the Threave Estate, owned by the National Trust, it is also the location of Threave School of Gardening and attracts students worldwide.
One of the features of the gardens is the number of varieties of daffodil, but there is still a lot to see all the year round. Being an estate it also includes, apart from the garden, farmlands, woods, a stretch of the River Dee, and there are wetlands catering for rare plants, wintering wildfowl and breeding waders which can be seen from "hides".
Warning ; If you have a dog with you and it is a hot day there is no shade in the car park for animals in the car, although there is dogwalks, dogs (other than guide dogs) are not allowed in the gardens.
Tourists Information will have details of other activities in, and around Castle Douglas