This game is complicated. But why does there seem to be the rule that the AI have to move all of their pieces every turn? They should be allowed to hold a position with their troops.
The AI doesn't hold a line. It holds a general area.
There should be subroutines written in... when attacking a city of a certain strength and era, there should be a way to offer probability of success based on a certain combination of troops. I can do that... I can look at an enemy city and determine that I'll need at least two siege, a couple of bows and maybe 3 melee troops to take that city. All told, without much resistance, I'll be okay. So AI should have at least that many before moving in...
You can look at a city and say, "Well, prolly need this, that, and the other thing." The AI doesn't learn like you do. Think back to when you first started playing and didn't know what you should use to take a city, so you probably overproduced a unit and underproduced another that would've been crucial to take a city.
After a few games under your belt, you feel you have a general idea what you want to do when you want to take a city. However, you run into a city where your previous strategy simply didn't work. Maybe its because the rough terrain means you have to bring your catapults to knife-distance to besiege the city?
Well, you can learn from that, but the AI...can't. The AI doesn't really learn.
Otherwise, yes. A good rule of thumb is to
at least bring siege equipment and bows, something so simple you think the AI would do that, but it consistently fails at that task. I've seen the AI build up an invasion against me pre-Renaissance that consisted of no cats, only melee units and one archer unit, despite the fact the AI in question could build catapults and maybe even trebuchets.
The AI is also notoriously bad at keeping defensive lines up. At one point, I attacked a catapult that was moving between cities. I did not destroy the catapult (couldn't follow up right then), but the AI moved it to the other city. Instead of allowing it to heal and, more importantly,
fire from within the city, it moved the catapult
again and I destroyed it with a nearby horseman.