This is a kinda followup from kain's post, since both his suggestions didn't seem like a good end result. This suggestion is great, yet all the crims will disagree with it, because they will still want some escape route after the failed attempt the first time. I suggest making some sort of a scheme where there the criminal still has got a way to get out, but not make the jail a DM fest. This idea cannot be ignored and need to be put to work! +1!