Bolsover castle was one of the first places we went that started us on our country house and castle seeing journey. It's part of English Heritage, so it was where we first signed up for an English heritage membership.
Bolsover is a shell of what it once was, with access to one part, called the Little Castle, that shows some of it's former glory.
When we went it was pretty gloomy, but the rain held off enough for us to explore the ruins for a while. It made the place seem much more eery as we read the plaques telling us what each bare room used to be and look like. They give out audio guides as well which are handy and definitely add to the experience a you walk around the grounds. You can find more information on their website here if you'd like to visit. I just had a quick peek at what's going on there, and looks like they are doing major works to redo some parts, and restore the castle walls so people can walk around them. Looks like I'll have to pay another visit to see it once they've finished.
Have you ever been to Bolsover castle?