Thornbury Castle provided this opportunity. I booked a bedchamber for the night, not completely aware of the delight about to come.
Upon checking in, I was escorted down some halls, through a huge wooden door, and started to climb a spiral stone staircase in a Tower, to my room for the night.
I would have been about 10-12m diameter, huge. Bed to one side, lounge in the middle, fireplace to the other side, with beautifully appointed ensuite and bathroom.
Windows on most every side availed views of the grounds in every direction. Comfortable, warm, and spacious. A large desk to one side enjoyed a window view outwards, and more than enough room for this travel blogger to catchup on some work.
The Castle itself has made use of every space to create rooms for staying. So aside from some common sitting areas and the restaurant, there was not a lot to explore, but as a Sleep in a Castle experience, it was brilliant.