I'll add another tip I didnt see mentioned. Train up 6-8 good assassins and a couple good spies. Send them into a town/fortress and let them reduce the buildings to rubble. Then move onto the next town/fortress. After you do this to a few of the surrounding enemy towns/fortresses they cant produce and field an effective army against you. Their economy will take a huge hit and maybe even experience rebellion if you kill their generals too. Then you can take them out at your leisure.