Originally Posted by
zoner16
Infantry in this game generally has low killing power but high staying power. If you have good enough ranged + cavalry play, you don't need to use infantry for much other than pinning the enemy in place, which you can typically do with just a couple of high defense units like Spear Guard. Infantry, even assault infantry, will generally not kill anything fast enough to outpace the cavalry engagement.
This is one of the bigger balance problems of the game, which is compounded by the AI's overcompensation in the melee department. It'll usually put more melee units into the grind than is effective, thereby leaving its backline open if it loses the cavalry engagement, which it often will, because it rarely builds armies like the player tends to. Most of its armies are infantry heavy, which is period appropriate, but a poor decision in game for the most part. The AI can react to cavalry attacks by peeling units off the line battle, but infantry is usually too slow for this to be an effective countermeasure if the player is paying attention.
If it kept infantry reserves around its backline, it would be more effective at warding off the player's cavalry attacks, but as it stands, it's main hope is to either win the cavalry engagement, or do enough damage that the player's cavalry is ineffective going forward. Again, this is rare due to the AI's army composition being subpar.
Now, if you ditch infantry completely or don't bring enough to pin the enemy properly, you could be in trouble. Polearms can brace against cavalry charges very quickly, and if your cavalry get pinned, they'll rout very easily under concentrated fire and infantry attacks due to their low unit count, so having your cavalry be chased by infantry can end up poorly, especially in Records mode due to the fatigue effects. Usually though, a retinue full of high defense units (ideally ones that can use formations) is all you really need to do this.
The player can make infantry heavy builds work with smart use of unit formations and a good army formation. The AI generally doesn't do these things adequately enough to be a threat with infantry builds. Sometimes, late game infantry under high ranking generals can be borderline unbreakable due to their gigantic morale, and charging them with anything but late game cavalry actually ends up not being very effective. This is usually the only time a decent infantry line will be under threat from the AI infantry, but this is rare.