One idea is that archers have a bonus when firing at your own wall from within the city. So thoose on the wall dont have a cover bonus other than the hieght when attacking down from the walls.. This would enable a more fun fire fight and the possibility of retakign the walls??
If the walls have small crenelations on the inward side, then there's no reason for archers to bypass this bonus. Depends on the design of the wall, the defender's archers aren't going to have a bonus just to make it easier. If you lose the outer wall, don't expect to regain it easily.