Battle Tactic: Open Wall Gambit
This is an is an early trick posted by others and occurs when you create a weakness or opening in a part of your walls to attract the enemy toward your legion which is pre-deployed there, or into a field of fire accessed by numerous surrounding ballista, or both.
If you did not build in such an opening to start, you can create one be deleting small wall sections, and the enemy following the path of least resitance will tend (but not always) to go there, thus falling into your ambush. If you do this, make sure you delete walls before the enemy begins their final pathing move on your city, or they will go to their original pathing destnation, no matter what you do to the wall.
If the enemy penetrates your city, the bounding overwatch-relay fire tactic with javelin troops works well, even in built up areas, but you'll loose buildings as the enemy moves forward.
See also: Death Row