To add to the discussion on Nuln, I'm not really worried about it falling anytime soon without Mathilde's intervention. The Elector Count is on guard for retaliation, the Emperor is likely monitoring the situation very closely along with the various College's. It was also mentioned somewhere that Elspeth von Draken spends half her time in Nuln, so she is probably keeping a close eye on it as well. Any skaven doing more than skulking in the shadows when she's around are skaven I don't particularly envy.