Lit only by scattered torches, The dungeon lies far underneath the castle.  Bandits and Thieves who find themselves there are met with cold stone walls and floors, with chains and bars restricting movement.  Guards patrol the corridors, keeping an eye out for any escape attempt or misdeed.  The only comforts afforded in the Dungeon are the piles of hay for sleeping.
Dungeon, Inside Castle
guard
bandit
I am a local bandit. I like to sneak around town. I like the forest a lot.
Oh, another simple foolish guard. Think yourself high and mighty, do ye? Indeed I do, what are you then that makes you so much better?  I'd like to think of us as equals if you didn't see it fitting to have me locked up in a dungeon. But I suppose you're stuck here too, hm? Stuck here? It's my job to guard this place, I'm not captive! The only difference is you are paid to walk these halls, but really, I bet I'll see the last of this place before you do. Oh you think so? I get paid and I am not here against my will. I bet you thought you'd be doing greater things as a guard. Ohh I do, this is just overtime!  You don't convince me. You should, I am the king's most vital guard! And he sticks you in here? He must be unhappy with you. I've just said that this is overtime, as in I choose to do this as additional work. Are you touched in the head?