|Don't fight your way through when you don't have too!|
In many cases when you want to conquer someone, there is no need of huge and costly army. First you should try to assassinate leaders, their heirs, etc. When enemy leader has a daughter - marry her! Kill all of his sons and successors and his country will be yours :)
When diplomacy or assassination failed, you will have to fight classic way. Try to use Blitzkrieg tactic ;) - concentrate all of your armies and attack one province. Remember, that your marshal should lead them and other commander should have high martial skill! That way you will easily win a battle and quickly end a siege! There is no point in besieging when your army is smaller then 1500 troops - the siege will take lots of time, and you loose troops that way due to unfriendly territory and low supplies. If you have an army of 3000 troops the siege is a piece of cake!
Ссылка по теме: коды на Crusader Kings