As is obvious to even the most casual observer of this blog - I am Cymrucentric. The top of my list involves things to do in Cymru (Wales). Although sleeping overnight in a castle anywhere would be fun, doing it in Wales would be even better. Wales has more castles than anywhere in the world for its area. If the English weren't building them to keep the Welsh down, the Welsh were building them to try to keep the English away. So now there 600 or so castles or remains of castles across a land the size of Massachusetts. That's almost enough castles for everybody to own one!
I've been to Caernarfon which walls still run through the city, Caerphilly (where there are working siege machines!), Chepstow sitting high above the Wye River, and a host of smaller ruins some of which names I can not remember. I have yet to sleep in a castle though.
There are some castles in Wales which have become accommodations for tourists. Craig-y-Nos, which I have driven past a dozen times, and have not been able to stop yet was once the home of famous opera Diva Adelina Patti. Castell Ruthin (Ruthin Castle) also offers accommodations and fine dining.