Incarceron by Catherine Fisher

Incarceron is a really, really good book. Finn, a Prisoner living in Incarceron, had always believed that he was from Outside; that is, outside of the Prison. He longs for freedom from the prison, and so a does a Sapient named Gildas. He doesn't believe that Finn is from Outside, but shares in his wish for escape.
Claudia, living Outside, is a girl set to marry the Earl, who will eventually become King. Her father is the Warden of Incarceron, and she finds a key in his study. She also becomes entangled in a plot that started with the supposed murder of Giles, the firt heir to the throne.