Well if another faction offers one of it's princesses to you, it means that if the other faction might die of , you'll get their land.

This also counts vice versa, although it's not like your going to care if the dratted Poles get a claim to your land once your heirless King dies, I mean the game end when your faction is destroyed !

So a handy tactic is to always accept foreign princess offers, then you can try and eliminate the other faction by using your assassins to kill of your allies royal line - and all the time they're thinking you're actually best of friends !

PS:line eliminated means that all your heirs to the throne have died off, thus rendering the game over - since you can't go on with hte game without a King.