Before I continue, I must first note that I play all my games on M/VH for maximum fairness. I have not noticed any significant improvement to the battlefield AI. I believe there are some small adjustments that do make a difference, but nothing that will really tip the scales in the AI's favor unless the battle is very close.
However, the campaign AI seems to be a lot more troop-happy than before. It will maintain larger armies regardless of it's economic situations, which I feel is often fustrating. I realize the AI did this in M2TW, but it seems to absolutely abuse it now, as you will often see about full-stack army for each city and castle the AI controls. After defeating the AI's front line forces, you'll often think they can't POSSIBLY have more troops, only to see a full stack defending each of their cities.
Campaigns tend to be polar, and either very easy or very hard.
In my experience, the hardest campaigns were:
By far, the hardest is:
Brittania/Norway - you are constantly, constantly strapped for money in the early game. You'll probably have to play a bit unconventinally to make up for it.
Some moderately hard factions are:
Teutonic/Lithuania - the tuetonic order starts in a strong position (lots of powerful units and has relatively hard-to-take castles), and with you pratically landlocked and with militia-like troops, you have to move fast and catch the order out in the open.
Teutonic/Tuetonic Knights - the main problem here is your economic position. Careful alliances will help to secure some of your borders temporarily, but the powerful order troops are a money sinkhouse, and unless you move quickly to capture profitible settlements, the order will be faced with many problems. The castle-centric economy means you can't grow large cities as economic powerhouses, which cuts down on the number of troops you can field in total. However, your troops are of superior quality, and that makes this campaign easy to steamroll once you get out of the initial economic sinkhole.
Any faction in the America's scenario - The AI really seems to abuse troop recruiment. I don't think it is actually possible (for a human player in the same position) to recruit the number of conquistadors (both mounted and dismounted) that I had to kill given the number of spanish-controlled settlements in my last apache game (only 5). The native american factions all seem to have nearly limitless supplies of troops, a good estimate of their power is usually a stack and a half per city owned.
The easiest Scenario is by far in my opinion is:
The crusades, especailly as KoJ, Antioch, or Egpyt - the more or less perm. alliance with the faction of your religion leaves one boarder permenantly secure, and allows all-out aggresive expansion against the opposite-religion faction in the area. The turks have to fight a two-front war with byz and antoich, and byz's spread out starting position makes logistics (and especially reinforcing troops) a small micromanaging nightmare. The crusade area overall is by far the richest and most well-developed of all the regions at the start of the game, and no matter who you play as, you start on firm economic ground.
Bookmarks