First, I like the way, how effective siege engines are right now. Especially on defender side. It means as an attacker you have to build some too, or your towers and your ram will be destroyed in no time. Nice so far. But in combination with retreating as a defender this means you can hold any castle against any force, no matter the size. Just run to your Catapult or Balista, fire a few shots on enemy troops (not the enemy towers or the ram), knock about 100 Enemies unconcious retreat and do it again. This way you can hold any castle or town nearly without any losses. An easy way to prevent this would be just to disable retreat. Or maybe keep it enabled but making it a try to break out the castle (with heavy casualties and the possibilty of getting captured of course). It just makes no sense to retreat from an attacking enemy and they just say "ok, if you don't want to fight, we're backing up ouresevels, giving you time to man your walls again and let you repair your damn catapult."