One of the things that makes playing the battles in MTW so interesting is the AI's use of flanking attacks and repositioning its armies to make things difficult for the player. You try and use good ground for cover, it will attempt to nullify the advantage the terrain affords you by approaching your forces from another angle. You try and do the same when it's on the defense, and it will manoeuvre it's army again to attempt to retain all the advantage it can. Sure once you play many battles you learn sneaky tactics and good ways to thwart the AI, but it can still give you a hard time.

This is in stark contrast to the AI in RTW which will gladly give up it's strong defensive position should you decide to attack it from the flank. I've had AI armies camping on a strong hill that I've approached from the side which results in the AI redeploying it's forces so they're downhill, facing upwards. Which is a sign of madness. It's amazing how much better the AI is in earlier TW games.