I don't know that we could say she did, though. She stayed in the castle the whole time, it's just that Kohaku went to her.You know considering even their queen fought on the front line they really should've seen this coming.
Assuming that holding that position indefinitely was ever part of the plan. This might be a suicide mission on the part of the lord; it won't be able to regenerate if its castle is destroyed, but I'm not sure the loss of the castle would necessarily mean it dies immediately. It might be able to keep running on "battery power" long enough to strike a fatal blow at the capital.Speaking of the Queen, one thing I think is interesting is that she mentioned her castle was the source of her power/life. So the Count leaving the place undefended could actually be extremely disadvantageous, if the Elite Squad is aware (or is made aware) of that little fact.