[RedRover]

Given time, I think artillery will find its way into game through mods and new nations.

I have a rather dusty Version of the Roman two-man ballista that I modded privately back when I was fooling around with some ideas for Pythium. No graphics, though.

My take is that we haven't seen artillery because its battlefield influence is too slight to have been considered before now. Any such unit would have a strategic move of 1 and would be completely immobile on the battlefield.

The tactical limitation of artillery is a huge reloading time. In a game where light crossbows fire every second round, a heavy arbalest fires every third round, and the board is only about 56 squares deep (making average closing time to melee maybe 3-5 moves for infantry), there is not a lot of room to play with reloading times.

Even giving 1 shot per 4 turns for the Roman ballista is generous, and that allows only one shot before melee.

Larger siege weapons fire a few times per hour. Not only that, but the big ones would have a pretty good minimum range, further limiting any battlefield usefulness. At a reloading time of 1 per 10 turns (very generous), you are still talking about maybe 5 shots in the longest battles.
And precision would be dreadful.

The strategic effect of seige weapons would be a bonus to castle sieging. For example, maybe:

Light catapult +15 siege points
Heavy catapult +25 siege points
Trebuchet +40 siege points

Cainehill: I don't much care for the strategic bombardment idea. This is medieval tech, not WWI+. Neither Napoleon nor Robert E. Lee had access to this type of capability. Effective historical catapult ranges were maybe 800-1000 yards, well under a mile, and far too short for province-to-province attacks.

On the other hand, freezing a tactical battle for a move or two so artillery can shoot (representing the approach of the enemy through the fire zone) might be an idea.

The artillery that I would like to see is the crude 15th Century gunpowder bombard and its fantasy spinnoffs (I'm a fan of H. Beam Piper's Down Styphon! novel and setting.) Also Warhammer mods would use direct fire artillery.

The big issue with these is "direct fire." Unlike all the current weapons, which pretty much loft their shots over friendly troops, if bombards (and by extension arquebuses and similar) don't kill units in line of sight, then you haven't got a good combat model.