Walls are pretty useless in the game, other than for esthetic reasons, but they don't provide much of tactical defensive advantage. Some suggestions to make it more useful:
- wall influence survival rates for defending troops
- wall have a 'delaying' effect on battles, basically moving more of the battle to subsequent rounds (effectively increasing chances of a draw)
- walls in combination with bowmen can provide an initial volley (defender bowmen hit attackers without hitting back, one round).