Re: How does the AI work?
 

As stated above, you need to have good PD, but I noticed the AI often send preemptive strikes when you move many troops on a common border.
It reacts rapidly when you expand too much, it is sometimes wiser to hold a chokepoint, and not to conquer the region afterward, so as to avoid multiple war declarations from your neighbours.
Recruiting chaff in border castles will have two beneficial effects : it will rise the apparent strength of your army, and block longer the AI armies (AIs always stop at a castle, even if they can't breach the wall)/