Battle Tactic: Tower Pickets
Build single separated towers at various key points throughout your city, and if resources permit, make sure their fields of fire overlap, throughout, so enemies attacking one will be shot by several others. These single towers are cheap, and besides picking off enemy troops at a rate of one per bolt, they will also cause the enemy to hesitate. I have noticed this repeatedly. This delays their attack, and gives more time for prefects, intercepting legions and additonal ballista fire to dispatch them. The slower the enemy, the more accurate the ballista fire. The enemy will be no more safe inside your city, than at the outer wall. Like the tower grid, but far more dispersed, the picket towers give defense in depth, unlike the single tower row along the outer wall.
These are great for negotiating with rioters or wolves too
See also: Wolves