I have no clue, since I think you need to ask this over in the warband section of the forum somewhere. In M&B, enemies overwhelmingly prefer to attack the weakest city or castle, so if your castle has 100 men and all the others have 200 or so, you become their best target. This is not taking right to rule etc in account, since that doesn't exist in M&B. The AI is a bit strange though, and filling a castle with peasants is just as effective at discouraging assault as filling it with sargeants and sharpshooters, but it's cheaper.