@Merkinball
Assume toned down cannon (eg. Bombard I described in an earlier post) at Gunpowder, but then Cannon as it currently is at Steel. Artillery is fine, and is followed by mobile artillery (reflects modern artillery). What bridging is really needed apart from the reneissance era?
How would you simulate "easier to move"?
I've added Bombard to Gunpowder for my own games: strength 6, +50% city attack, ignores walls/castles like cannon - basically "a bit better than treb against cities, a bit better than cat outside cities, so better than either in both cases, but not as good as cannon". I think the city attack bonus with lower base strength currently simulates "hard to move and use on field - best used against stationary target in sieges".
Sure you can delay artillery, but that doesn't mean there's a gap between Cannon and Artillery (unit). Just that you don't see the need to jump to the modern era siege weapons - maybe you don't wage wars during that time period?