Within moments, depending on your speed setting, this city will be attacked by 45 carthaginians and 5 elephants. Sit back and watch the show. The javelin legions are small but effective, I still haven't figured out how to do multiple barracks! And wall guards really do fight!
You should note that all approaches from the north and west are covered by two sets of walls, with plenty of space between for a redoubt. The outer walls to the northeast were breached by the earlier invasion of 60 soldiers and 7 elephants, but that's about as far as it got. this double set of walls is the key and should be in place before the big attack of 108 soldiers and 12 elephants. You should also note the positions of the Javelin legions. These guys are in firing positions. Also note that the inside ballistas do fire at close enough targets.
To see the enemy attack a different part of the walls, move the snake legion, inside NE, a bit further north - into the corner, but still inside the walls. Also, if you don't like to let your enemy escape, use the snakes to cut off the avenue of retreat after the enemy has committed to the west wall (and after the elephants are gone). You can also use the stallions as support for the snakes.