I just had a little mess around with a unit of ballistae vs. a unit of my own armoured sergeants; unit sizes on large; weather clear; morning; VH difficulty.

On Grassy Plain the enemy would always advance to point-blank after they'd reduced my unit to around 72 men. Those AI fellas love a bit of point-blank, gangland execution style huh?

So I thought by having a forded river in between sides, the ballistae wouldn't be able to advance. I used the River Jordan map and that worked well; the enemy stayed put throughout and allowed my lot to advance into range exhausting all of their ammo.

There is the question of how many ranks deep you deploy the defending unit, as projectiles always hit the central files and sometimes take out a whole file of men at a time. This means that early shots have more chance of greater damage as later shots are only fired at a single row of men. I suppose this effect would average itself out though across many tests.

Secondly, you have an effect where the defending unit creeps backwards gradually over time so you have to watch that you start off well within the ballistae range to avoid intervening by moving your men forward mid-test.