The checker fort formation I believe to be unnecessary. Build a fort where the enemy has to pass to get to your city. FILL it with units. Have more than half of them missile units, a couple of good defensive units, a couple of good mobile offensive units and some cavalry. Missile units are relatively cheap. If an enemy lays siege on the fort, sally and decimate him. If he tries to bypass, exit the fort and do battle. Make sure you have reserves withing marching distance and always replenish losses. Crushing local superiority will ensure that your city will not be bothered.