Crusader Kings II

[Monk]

It's a constant balance of power. You want strong vassals because they help lessen the micromanaging aspect of the game, ensuring your realm is growing steadily along with your demesne. But.. you can't afford to let them get too powerful, or they will rise up to challenge you.

Never think for a moment that your vassals are friends of your ruler. The military AI can be a bit suspect at times but the character AI is good enough to know how to advance its own interests. If you're getting your ass handed to you on the frontier don't doubt that your enemies will take the chance to declare a faction revolt while the levies of your supporters are down.

Vassals are the rungs on the ladder to success, don't hesitate to step on them. If a duke gets more than one duchy title, try to scheme one off of him. Try to assassinate, plot, or otherwise reduce his power. If all else fails use your chancellor to fabricate a claim on one of his duchy titles (Expensive but can be worth it). If you have a strong claim on a vassal title you can revoke it for free.

And remember most of all: If you have to injure a man.. make it so severe that you'll never need to fear his vengeance. Never leave retribution half finished.. you rule a medieval court of intrigue, not a social club.