My memory may not be the best on this but this was how SW based orc armies did so well, they wouldnt lose enough on defense to lose the fight.
In this case there are two ideas here, no one to take the said castle/land OR no one left on the land therefore the attackers people move in.
Personally i think if both players 0 eachother the defender should win. I just dont want the game to turn into a farm fest cause attacking costs more than the benefit vs farming.