Once your faction is at war with the crusading faction you shouldn't have a problem to rush to the defense of the castle.
So, you're at war with the crusading faction but the faction of which you're a merc isn't at war with the crusading faction? Then you'll have to loot/attack whatever there is in order to provoke a war between your faction and the crusading faction. If you're a female you'll have a hard time finding a husband, life for females isn't that easy in warband as it is for males.
I only know of the possibility through the wife, don't know if the chancellor also has this option if you're not married, because as soon as I am a lord to some faction I will go after the ladies in order to get married.
