Battle Tactic: Tower Grid
Create a grid of road squares two by two each and place the array in front of your city, or near a battlefield frequented by your field army. Build a 2 by 2 wall on each grid square and top it with a tower. The road grid with towers should form a large square of units, surrounding a small housing center in the middle with four tower rows extending out on all four sides. The housing can supply at least this many rows of towers with employees and guards. There is no continuous wall, just a grid of closely placed towers.
When the enemy attacks they will usually not tear down the towers, but will follow the path of least resistance between them and take ballista fire all the while. By the time they penetrate a few rows they usually are ready to retreat, leaving little damage, (One of two towers at most.)
The tower grid can also serve as a base of fire behind which your army can retreat and also from which your javelin units can sally out in repeated hit and run attacks. The grid gives depth to the tower defense, and is more effective than walls, which have only one row of towers.