Welcome to the Org, Tancredo.


First off, I strongly encourage you to read this post by frogbeastegg if you've not already done so -- the section on artillery is near the bottom.

Otherwise, the main piece of advice I can offer is to remember that when employing siege weapons against larger fortifications (especially Citadels & Fortresses), you should generally try to place them close enough so that they can take down the castle's inner wall(s) in addition to its outer wall. While this probably will expose your artillery crews to enemy missile fire, it's usually a worthwhile trade-off as otherwise your main assault troops will be stuck trying to break through the inner gates/walls with no artillery support (which is nearly always very costly). Better to lose a few artillery experts than a few hundred medium/heavy infantry!

Unfortunately, I can't say exactly how close/far away you should be (especially since it depends on which siege weapons you're using and how many walls a castle has). I invariably wind up just going by trial and error more often than not.