Crusader Kings lets you control any kingdom, duchy, or county in Europe from the time of the Battle of Hastings all the way to the early fifteenth century. As the leader of one of these provinces, you will set taxes, appoint counselors, raise armies, build an infrastructure, marry off your children for inheritance purposes, and try to cope with the rest of the world as they look out for their own interests. The strategies you can employ are boundless, and the outcomes branch out just as much. On top of all that, the developers that make this game also made
Europa Universalis, meaning if you survive to the end of
Crusader Kings you can import your game into
Europa Universalis and keep trucking for another few hundred years.
The very beginning of the game will give you an idea of how deep things can get in Crusader Kings. There is a list of kingdoms, counties, and duchies you can start as that's as long as my arm. You can rule as king, or you can serve under the king of any country as a Duke or Count. The difference is the number of provinces you have to manage. As a count, you only have one, but you may be called upon by your king to drop everything and go romping around the countryside slaughtering his enemies.
On the home front, you must appoint your counselors from your pool of courtiers. These counselors will help you in war, diplomacy, espionage, economy, and religion. There is virtually no limit to the number of people generated in the game, and each person has various traits that will affect how they act. If you appoint a scheming Marshall to look after your armies, or a traitorous treasurer, bad things may come.
Structurally, you must impose laws, fix taxes, build improvements, and research new technology. This is the most intricate part of the game, and if you don't have a strategy planned out, it's easy to get in over your head. On the other hand, if you have a general idea of what you want to do, things start becoming very intuitive and it gets easier to feel your way through it all.
The game becomes a whole new experience when you get online. 8 people can go through the entire timeline, and each can play as any of the people in the single player game. This gives Multiplayer at least three different levels of depth, with lower Counts and Dukes possibly joining forces to fight a stronger player who is a King.