We have had the great luck to visit many, many castles over the years. Threave Castle is easily one of the most unusual. Built on a small island, it is only accessible by boat.
After parking in the designated car park, it is about a mile walk to the bronze bell, which calls Scot, the ferryman. Scot then takes you across to the castle via outboard motor.
The castle itself, though built in the 14th century and inhabited only until the mid-1600s, is in wonderful condition, with a Great tower with rounded ceiling, many fireplaces, and the stairs still access the Great Hall.
As it was a rainy day, we had the whole place to ourselves, which we enjoyed immensely.
You can easily see old doorways and fireplace openings.
Travel Buddy stands 5’2″. So that gives you an idea of the size of this opening.
The Tower, and looking up through it.
This was one of the more unusual castles we’ve visited, because of its unique setting. A definite recommendation if you’re in the area, which is near the town of Castle Douglas, in Dumfries & Galloway.
Wordsmith Magic 
What a neat experience. I love castles, but never had one to myself.
I am absolutely sure the weather contributed to our unique experience. Thanks for the comment!