The Oxford Casle & Prison is a very interesting place to be. You can only get into the castle by a guided tour. The tour guide will lead you up the castle tower where you get a good view and see the city from another perspective. After this 1-hour tour you can explore an exhibition about the prison yourself.
To be honest, without the tour and all those exciting (but also a bit horrible) stories which you get to know meanwhile the castle would not be as interesting to see.
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