I was just playing normally as the julii when i came across this. I thought it was a lost cause, i nearly clicked to let the computer do the losing, but i felt suddenly compelled to lead a suicide mission. To my immense surprise and pleasure, i managed a stonking great victory. 'Heroic' was very adequate.
You start off with post-marian army of 400-ish. The carthaginians have countless siege engines and around 3500 men. When i did it, i ended up with 200, and had killed 3000 carthaginians.
Theres only one tactic (that i could think of) that will get you through this alive. If youre stuck, ask me.
Fairly easy to win. Once you get the men doing the right things.
*Warning spolier below*
The rams never make it to the walls in my several goes, use your heavy infantry to defend the wall against the towers that attack from the rear. (as you get a bonus and your infantry is way stronger). Remember to keep them well out to the side until the towers reach the walls to stop them using the missiles on you.
Use your cavarly at the other side to take out the users of the left hand wall tower, you should be able to stop this for a while and it will cathch fire (as soon as this happens forget about the infantry). While doing this take your skirmishers outside to take down the ellies.
By this time the General should have attacked your cav and you should be able to roll up the cav line by attacking one by one.
After killing (most of) the ellies move your skirmishers back inside to defend against the last tower. Any unit that arrives will have been under heavy fire from the wall towers so should be down to half strength also use your remaing cav to try and break/damage the weaker units on thier way to climb the tower.
Once this has all happened the battle seems won but not quite you have to get rid of those numidian cav in the rear. Haven't found any better way other than moving your infantry just outside the gate and taking the casulties until they run out of missiles and charge. If you put fire at will on you may be able to catch a few of them. If you have any cav left use them in a big sweeping arc to try and drive the numidians onto your men.