Where can you rappel down a castle wall?

I know that overseas (such as in the UK and Europe) that you can rappel down a castle wall - I was wondering where could you do this? (As in country, then city, and which castle). Also, how do you could go about doing such a thing and how much does it cost to be involved in such an activity? Do you *have* to be part of a large group to do such a thing? (I know some programs abroad programs do this as a recreational activity). Thanks in advance!