Of course it's a Succubus. Only succubi are kept in dungeons. Damsels are all kept in towers. Unless the succubus also happens to be posing as the damsel from a well known merchant family, and has been doing so (through the guise of shape changing magic) since the real princess was killed only days after her birth. Of course, the family doesn't know about the switch, and they have no reason to suspect anything being amiss, after all, the "damsel" certainly never openly acted evil...
There's a moral quandry for your paladins! :-D