I went to Oxford Castle Unlocked recently and was in fact rather surprised by how good it was. It's not particularly cheap, with adults' tickets at £7.50, under 15s' tickets at £5.35, and concessions' at £6.20 (although if required, one carer will get in free per fee paying guest). But the tour is certainly very interesting. A guide (ours was the Empress Matilda) gives you a historical tour of the castle, telling you the ghost stories of the dungeon, the modern usage of the castle as a prison, the story of how King Charles used Oxford as his base at the time of the English Civil War (he lived in Christ Church; his queen, in Merton College) and a trip up to the tower, where you can see a large portion of Oxford. The guide we had seemed very knowledgeable, and she was happy to answer any questions we had. Once the tour is finished, you are free to explore the prison side of the castle, which has exhibits and displays about prison life.

The attached shop sells all sorts of paraphernalia to do with the castle, as well as the customary swords, children's history books and so on.