I like to go for maximum killage, because even if the survivors are so few the siege lasts forever, at least they won't be killing my men in the meantime. Attrition to the sieging army is determined by total numbers inside the castle... and a small garrison can often be bribed... or the doors can be opened on it by a spy, after a few years. If you choose to assault and you have the appropriate equipment, losses are usually not so bad.
Also, if you get them all before they get to the castle, then they won't be able to sally next year with outside help and kick your butt! And, if worse comes to worse, you can always retreat and then re-attack with a small unit or two, and this time get ALL the little flockers!
DA
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