This is a tactic employed by the AI that i find very strange?
For example, say a province has 600 men and is attacked by 1000 of my men and the fortification is castle-sized, which means it is capable of holding 600 men. The Ai in this circumstance will more often then not retreat to the castle.
Firstly this allows me to pillage the province uncontested.
Secondly, the length of time they will be able to hold out in the castle will only be 1 year from the following turn due to the sheer number retreating To me, castle siege times are still too short when the castle is near capacity.
If there is no relieving force, then this is nothing short of AI capitulation as it'll lose 600 men in 1 year for almost no cost to me. IF there is a relieving force, whether they will be able to do any better than the initial 600 v 1000 is debatable since they are attacking me and i now have the advantage of being the defender. More often than not though there is no relieving force.
Now....
With that number of defending troops, imagine they decided had defended their province whether they knew they were going to lose of not.
Firstly, if they had lost they'd have certainly have killed many of my troops in the first place.
Secondly, they'd now have a lot less than 600 men in a 600-man castle, therefore resulting in the approximate siege time being considerably greater.
What all this essentially means in gameplay terms is that i have now lost many men, face a longer more costly siege and as a result of both of these will be subjected to a greater likelihood of loyalist revolts in the besieged province.
That's my thoughts.