I finally visited the 'Unlocked' attraction at the Oxford Castle yesterday, and thoroughly recommend it.
It is a history of the castle as a prison and a castle spanning hundreds of years, going back to Saxon times. You are shown around part of it by a guide and then have a handheld audio visual unit to guide you at your own pace through the prisons life through the ages. Good for all ages with a few interactive things and actors bringing the stories to life on the AV units. There are also good views of Oxford from the top of the tower
The cost is £6.95 and you need a good 1.5 - 2 hours. Plenty of refreshments outlets and now a very nice, small open air ice rink.