Yup, no phalanxes on walls. They are way better in the streets. Just deploy your archers on the wals, if you have them and let them rain arrows on the enemy, not forgetting to take them off the walls and to the town square when the enemy gets his ladders/towers at the walls or when he is about to breach the gate. Create a good defensive position somewhere near the town center or square, whatever the name, by putting the pahalnxes one behind the other. Take you cavalry, should you have any, and prepare to circle the enemy that's bogged down fighting you phalanxes and attack from behind. Watch out for towers shooting at your cavalry. That's how I play defensive...