This issue has been bugging me since the 0.9 version, and I'm not happy to see it still being in Warband. Conquering cities is really hard, but if you get the defenders to sally out, it's very easy to kill them. I'm here a level 14 guy with Great Axe. I'm playing with player dmg easy (not easiest, though) , Allied damage easy. But still compared to the pain of conquering the city, this is ridiculous: