The place where criminals, thiefs, brigands, bandits, beggars and pickpockets are spending their relaxation time for X days. Remember, a criminal have chances of being caught in castle and towers. High bounty raises the chance. Also, a criminal can be escorted to jail immediately for a certain failed criminal actions ( begging, stealing book).
In jail, there are four actions, namely:-
(a) Spend your time for X days in jail and accept your punishment. The easy way to get rid of your criminal status but wastes time.
(b) If you want to be in a hurry, just escape. Rogues get bonus in escaping. But, escaping from prison is not a fine bet because it raises your bounty. Also, failure in escaping will get you in some rough handling (lose 1 hp) from castle guards and additional jail time.
(c) Bribe the guards 100 gold but this is practically a gamble. The outcome is random, the guard always took your gold.. in a fine situation, 5 days but worst case scenario is 15 days in prison.
(d) Profess your innocence and this is based more on luck, and gamble. The character have a basic 5% per level, if you are charming give +3 bonus. Success will cause the guards are convinced by your so called sad story and they will release you. Failure will cause the guards to be pissed off and increase your jail time.
Just pray you are not caught when you have poison and disease in your body. A safe bet is to always wear a mask to avoid detection.