Great ideas guys.
As a followup question. Since a prison is built like a fortress, it will prove difficult to adventure in the traditional sense. What types of quests can a party do within the prison or under close supervision? For the sake of discussion, consider the party low level and assume that they won't spend their entire lives in the prison (maybe a few sessions).
Explore and mine -
see Wieliczka Salt Mine - Wikipedia, the free encyclopedia
see http://en.wikipedia.org/wiki/Chełm_Chalk_Tunnels