Cost per tile: 250
            Room Health: 600
            Monsters it can attract: None

 The prison can be used to hold troublesome minions or captured Heroes.  Imprisoned creatures will eventually die (unless they can heal themselves or you heal them), and may return as skeletons if they do. However, this is not the best use of your prisoners.
 Prisons can be used as free (and extremely effective) training rooms. Here's how:  Build a large prison, and always have your minions set to "imprison" during battle. Make sure you have enough Imps to transport unconscious foes back to the prison. Now, drop some creatures into the prison. They'll fight any enemies located there, gaining far more experience than they would in the training room - and it's totally free, apart from the cost of the room. This is most effective with higher-level creatures, who take much more time to advance levels and cost more to train.