Crusader Kings in many ways is a biographer's wet dream. It has mechanics of Europa Universalis 2, the warfare, the economics, the diplomacy but unlike that grand strategy epic, this grand strategy epic portrays you not as some nameless entity ruling your nation through the ages but as the head of a noble medieval family, your faith not tied into the success of the nation you rule but the rulers themselves.
You start the game by picking which family to control, you can start off as a great King yes but you can also choose to play as a powerful Duke or even an impoverished Count. You then set about forging alliances, mostly through marriage, produce offspring that can one day take over the mantle of leadership and assert your claim to various noble titles. As you grow in power and influence new challenges will arise. Conquer too quickly and others will see you as a warmonger, your vassals may grow too powerful and seek to claim your titles for themselves and should you have too many children they may differ over who should rule when you are dead.
Each character has a set of characteristics and traits that wouldn't feel out of place in an RPG. Some traits are purely positive while others make you an inept ruler. Each trait also has an opposite, brave to cowardly and so forth and characters with opposing traits will often dislike each other and this all works together to create a great web of intrigue that makes holding together a great Kingdom a real challenge.
This great seat of intrigue and dynasty building is not without it's flaws however, the lack of any sort of proper tutorial and the rather extreme wealth of information you get showered with from the moment you start up the game makes it near impossible for newcomers to get into the game and even veterans of Europa Universalis 2 will find themselves struggling to keep up with the flow of information.
All in all Crusader Kings is an extremely complex dynasty building game that more often than not throws you into the deep end and see if you can figure out the diplomacy of marriage politics before you die without an heir or have to resort to incest. It is a fun game, once you learn it but unfortunately most people will quit long before that.