Quote Originally Posted by human_male
So I slaughter them on the field but the French just keep coming, and the Danes just keep coming and I get nowhere.
I'd advise slaughtering them in their towns and castles instead of in the field. You grow while their ability to produce troops shrinks. Go straight for the jugular.

Lots of good advice here on economics already. If you can't afford to steadily build troops in your castles, there's little value in them being castles.