Gentlemen (and ladies), behold!
I was able to create an approximation of Rome Total War: Barbarian Invasion's Carriage Ballista from Bwian's Chariot resource.
Since it is based on the elephant artillery unit, it does not have the ability to skirmish, nor could the turret rotate, making it a lot less powerful than its original incarnation.
As this is meant as a refined add-on to Bwian's chariot, please follow his directions via his thread. This include some updated skeleton texts for the chariot with basic movement and death sound effects. Unit cards not included.
(This was originally meant to be a discussion thread for implementing the unit, since it was completed, a mod could move it to either the resource section, released mod section, or wherever appropriate)