I have to say its the defences that give the defenders the edge, rather then state boosts. First the attackers get peppered by what ever archers there are on the walls and by the towers. When they do reach the walls they can at first only enter and be attacked immeaditely, if that didn't happen then I'd say about the first 10 men that go on the wall wouldn't die so fast. I have lost wall battles many times even with superior infrantry, so I'm left to believe that there isn't any boosts.