First make a war scenario, for the first mission you can do whatever you want (I chose for the victory condition to be to kill the lord of the castle I intended to occupy)
Second, add a second mission, and for the trigger set it to Always, then set the result to "Take Enemy's Castle." You will be prompted for which castle, I chose the same lord that was the victory condition in the previous mission.
That is it, after you successfully complete your first mission, you will automatically switch castles to the one you chose in Mission 2. The castle you had in Mission 1 will now be a friendly castle estate, that you can choose to have economic, defensive, or aggressive stance.
Hope this helps someone (anyone can feel free to make a better structured tutorial if you wish)
Here is a teaser pick of my map still a WIP, back story to follow later, but essentially you are the remnants of an army hiding in a forest. And the big bad fortress you eventually have to conquer (the last objective) [This message has been edited by Redeye CoV (edited 06-20-2005 @ 08:19 PM).]