Furthermore, if possible, don't limit yourself to just one castle. Like a bliz, try to conquer two or three.
If you just take one castle, the AI will come in force to retake it, and you will not be able to keep it... Gain Zero.
If you take multliple castles, you just can play ping-pong with the AI. They will retake one of the castles, leave a small garnison there. You retake it, the come back to siege it. During all this time, the other castles remain in your possesion. Gain ... over 9000.