That's one thing. Secondly, new artillery won't give you anything... except control over the ballista. But if you take advanced fire and +1 spell power, the wall will deal 80 damage, which means you can roast all the elves except the magic one...
In general, there's a much simpler and more beautiful way. And I did break them down in Tower. True, I didn't take Tower units into the army... :).