Don't think I don't know this game. Wall towers don't benefit from the Crenellations tech, which gives 40% more arrows for each garrisoned soldier and is basically necessary for effective tower usage. Also, regular towers have significantly better range. HP doesn't mean a whole lot. If you can't take out enemy siege equipment it doesn't really matter whether your towers have 1000 HP or 5000 HP. Current wisdom is to use fully garrisoned regular towers and fortresses in conjunction with archer balls to take out enemy spearmen so that your melee cavalry have easy access to enemy siege equipment. Catapults have greater range than wall towers, but not regular towers, so wall towers are not particularly useful for defense. There's a reason competitive players rarely use walls. The only way the AI can get walls to begin with is if it's playing as the Iberian civ, in which case it might delete it's towers to avoid the problem of pathfinding around gates (units tend to get stuck). Basically, the AI doesn't support walls at all right now. It can't build them and if it starts with them they only hinder it.