The thief would find themselves unable to open the door - the next day she'd spend time going through corridors, trying to open various doors when the guards weren't peeking.  The first few locks she managed to sneak up to didn't budge to the key - but she did figure out more about the layout of the castle.

Straight ahead were the main entry areas and the main hall - located conveniently where the lord might be receiving guests - along with the kitchens and other amenities that would be necessary for such a thing to function.  To the right was the area where the nobility seemed to stay, but the curious woman wouldn't get far - there was always an alert guard on duty there.  Perhaps it would be worth sneaking past, but if the castle itself was any indication there probably wasn't much worth stealing.  She'd hear Zarva's wonderful voice, though.

To the left was the area where the commoners employed in the castle stayed - barracks for the guards who worked in the castle, a few more guest rooms, and a small, damp room unworthy of human habitation.  A small distance from a cell door it'd be easy to see racks of cots going from the short ceiling to the floor with only about a foot and a half of head space.  Perhaps twenty slaves were crammed in the room, largely unmoving as they rested from the day's labor.  She'd likely recognize the thirteen year old on a rack near the door sleeping with her face buried in her arms.

Eventually she'd find a nice looking, well maintained door down one of the subhallways in the center - the oak the door was based in was well made, if not opulent.  Without any guards looking, she'd pull out the key.

Her hands would come up empty.