You are right of course. I have already played the campaign and won and and I have been playing this game to get the most out of defending and attacking castles. I have been using one large expeditionary force to plod about capturing provinces and relieving those under seige - hence the questions about garrisioning.

Anyway, this tactic, although a nightmare at first became a winning formula once I had control of the sea and before long all had fallen save the Byzantines in the southern provinces. However, due to my lack of expansion during the Early period ( no conflict = no gains ) by the ten turn ultimatum I could see that I was going to need to claim at least one province per turn to win. I gave it a huge push, sending all those seige-veteran garrisions to to front line, throwing assasins at heirs etc. Just as I reached the the end of the campaign civil war split my empire, my main army defecting and near all my provinces undefended. There was just enough game time left for the rebels to rub my nose in it before the game ended and I was shown the hitherto unseen "You Have Lost" screen.

The campaign was great fun to play though, and my units did some Spartan style defending at times. Billmen proved to be a winner in seiges. Just one lone horseman can destroy a seige engine and get back within the castle wall. Organ guns are not much good against expert troops either.
The AI should have finished me much earlier, but maybe the fortresses put them off. The expert AI performed much better in Seiges too, not just standing around getting peppered till the time expires as I have seen them do on easier settings.

I never did get those bloody bandits either.