Only I go to prepare the new, even harsher prison, and what do I see but kitten poo next to the litter tray. I'm not certain whether or not that was there when Merlin went into solitary. Also, the new mess appeared when Nineve was visiting kitty prison. So now I'm not sure if it isn't Nineve who's the culprit.
So for the moment, they're still in the library (Nineve hasn't shown any signs of wanting to leave today otherwise I'd have let her out). Gods why do cats have to be so cursed difficult?