The last game to come from Paradox Plaza was the absurdly complex Victoria. With an interface that should have come with its own Google popup blocker and an economic system that put the player right in the thick of things, it is without doubt one of the most complex strategy games ever released. One would assume that Crusader Kings, based on the Victoria engine to some extent, would follow that with even more complexity or at least the same level but with a streamlined interface.
"One" would be incorrect. Crusader Kings is arguably one of the simpler games to come from Paradox. Its game mechanics are ostensibly more complicated than in the Europa Universalis games - after all, you're managing a dynasty, its vassals and its possessions, rather than just a nation - but we did find it to be a simpler experience.
For starters, the player doesn't have to run a kingdom. He can settle for any of the numerous Duchies or Counties in the game. After choosing a time period (there are three, but all end in 1453 and the earliest starts at 1066), the player can toggle between lists of Kingdoms - a short list, Duchies - somewhat longer, and Counties - an overwhelming number. What's the difference though?
Well, keep in mind that you do rule a medieval dynasty, not necessarily a kingdom. This is the age of feudalism where peasants exchanged labor for protection from knights who pledged their service to a local Baron for mutual protection who in turn did the same with a Count, Viscount, and they bowed before Dukes who kneeled before Kings. In order to maintain control over their realms, Kings would have to give the Dukes enough power to satisfy themselves but not so much as to make them independent-minded. One of the best ways to ensure loyalty (or claim land) was to marry off daughters to powerful lords or to bring their daughters into the royal household.
This is, in fact, all simulated in Crusader Kings. The family, the title, and the land are all equally important. A King can have Dukes and Counts pledged to him, a Duke can have only Counts, and a Count is lowest among equals. The titles are important since if you're a Count whose realm is getting large and inefficient for one ruler, and you create another Count hoping that he'll be a vassal, you're out of luck. He immediately becomes independent.
There are three ways to gain titles: to inherit, to create, or to conquer. Say you're Count Blah or Blahcounty who is pledged to Duke Duh, and you married the Duke's third daughter. His sons die, he has no brothers, his other two daughters die… and your son becomes the heir to the Duke. As soon as both the Duke and your Count die, you take over the entire Duchy. Titles can also be created if you have 2/3 or more of the appropriate land. While there is no Duke of Romagna at the start of the game, if you grab the right lands, you can create him. Finally, if you have a claim on a title, you can declare war on its owner and demand it from him. Deep enough for you?
Every character in the game has his own statistics, abilities, parents, probable spouse and children as well as the claims associated to him. A popular activity among Crusader Kings players is to get not the girls with the best claims into your family, but those with the best stats. This is affectionately known as the "Kwisatz Haderach breeding program" in reference to the almighty Paul Atreides of the Dune books. Most players, however, seek advancement and the best way to do this is to get the daughters of weak dynasties - those with few or no sons. Hopefully, nature takes care of the rest… or at least can be given a helping hand with a few hundred gold pieces and the heir apparent might be found in the morning have accidentally brutally disemboweled himself while shaving.
All the player's children can be given the choice of a court or ecclesiastical education, and in the case of male children, a martial one. These improve their statistics, and can lead to traits stemming from that education. The variety of characteristics is bewildering, and some of these are hereditary to an extent. One branch of my Italian family has a strong streak of insanity running through it. Characters there all fall into deep depression and often various manias. If given the opportunity to run a county, they'll often engage in lunatic acts like rebuilding the tower of Babylon. While never permitted to inherit the throne, their insane actions do interrupt the relatively common slow parts of the game to provide needed comic relief. There are simply too many times I've found myself waiting for something to happen if I am in no position to start action.