Tecumseh1
Prince
This is what the AI does so it's automatically suspect.
In this specific instance it's probably best to spend a turn reducing Chicago's defense to 0. But there are lots of situations where you simply can't wait to bombard each city to 0. That can take many turns pre-gunpowder if the city has a wall.
I'm curious how other players conduct their cannon wars. Generally I try to send some cannon for pre-bombarding while the main stack is healing. I'll bombard the city closer to 0% but won't skip a turn of the main stack's movement to wait for more bombardment.
Bombarding to 0% might save some cannons, but all sorts of bad things can happen when you fight a war too slowly.
I will rarely spend a whole turn bombarding defenses. It's usually less efficient and too slow. Esp with cannons (or trebs even if they don't have too many longbows). I will bombard with a few, the promote a couple to collateral damage and suicide. Then use city attack promoted siege to further soften. And can take city in 1 turn if you bring enough units. Of course that isn't always a luxury. But I'd rather suicide a few siege and take targets more quickly. I usually don't split my stack or try to bombard 1 city ahead. It's too tedious to keep things tight and usually bad things happen to me when I don't use concentration of force.