Hmm...
Thank you both for the feedback.
I hear you about the lack of guidelines. Going in I figured it wouldn't be too important; I figured we'd be able to "roleplay it out" and I was actually worried that putting in place firm rules would overcomplicate the equation and bring everything to a screeching halt. Instead what happened was the game evaporated into vaguaries.
As for the Carthaginians... that's difficult. The reason being that the purpose of the game is to play a team controlling the losing side off against a living, breathing rendition of the winning side, rather than against a static strategy that anyone can defeat with hindsight.
I have a few ideas... one involves setting up a squared board of the battlefield, with each square the same distance across in all directions. The other involves giving movement rules to each individual unit type; mixed groups of soldiers would move at the speed of the slowest unit if the unit types got jumbled up for some reason.
That would at least give some direction to the battle.
While not so important to Cannae, there would need to be some terrain rules. And, of course, combat interactions. I'm not entirely clear how to go about those yet. On one level it's easy; spears beat cavalry, archers shoot but lose at melee, phalanxes collapse if attacked in the flank. On the other hand, how do you factor in fatigue? The time it takes each unit to drive another off?
Beyond that you've got the stuff I've labelled in my head "advanced". Wind speed, things like the sun getting in the archers eyes, the effect of morale and organisation on performance... there's quite a lot here.
I'll give it some thought, but for the moment I'm chucking ideas out as I think of them.