Provinces With Castles

[Leblanc]

I was recently playing a game as Abysia and in several provinces with castles I had my mages hunting for blood slaves. The problem was that every time any of these provinces were invaded, no matter how big my army was it would retreat behind the castle wall and the mages would have to stop hunting for at least another turn until I could get the invading army out of my province. This put a bit of a damper on my blood magic plans as Caelum was flying in and casting whatever spell sends an army of Spring Hawks to target province every turn and I could rarely get more then 10 blood slaves. My question is this: is it possible to make it so your army will automatically defend even if a castle is present? I could just be doing something wrong but it is quite frusterating.