Any themed room design that contains a prisoner NPC should also contain a strong, nasty monster who is guarding them.

Special “cell” room. Only valid if one end of the room has no doors. It generates a line of iron bars across this end of the room, with a locked (iron if possible) door somewhere in the line. Independently, it may contain: a prisoner, a sink, a bed (if those exist), and 1-2 chests with random contents.