As for the auto resolve they greatly improved it for annoying battles. For battles where you have 5-1 odds or greater you will usually actually take slightly less casualties than you normally would but also kill a lot less than you normally would. So slightly less experience at the cost of saving a few men. Just don't use it when the odds are close.

As for defending walls theres a few factors. First those archers probably arent making to many kills once they engage in melee, what is really happening is your towers are taking them out while your archers are there just holding them in place.

Taking the towers out of the equation, siege towers lets quite a few men out in 1 swarm so they should win vs archers. Now as for ladders your men are coming up in single file so at first they are getting double teamed at first until the unit looks like it gets a foothold. The hard part is getting men on the wall instead of ladder fighting. Also there just might be a penalty while fighting on a ladder, I have no proof but it does seem elite units fight poorly off a ladder. So ladders are best for unmanned sections or to take inner walls, or if you have very few build points.

Also Fortress/citadel are quite deadly no matter what you send up the wall.