[HJ]

To try to contribute a little to the discussion.
I was thinking about the AI pacing back and forth in the border provinces. I don't know how the AI handles that situation now, and whether it's actually reacting to the border and neighbouring armies by amassing troops but then running out of supply in that particular province and hence moving the troops out. But to make it a little less predictable, maybe something like this would help:
IF (enemy army present in neighbouring province)
THEN (stay in the province for 1+x turns if not starving, where x is a random dice number)

It would seem that the AI knows when it's threatened, judging by the amount of local defense it puts into threatened provinces, and it should know whther it's out of supply as well, so maybe this would work if it's not already working this way, and the only problem is the supply route, which brings us back to the castle issue.