Europe is in turmoil. The lands are fragmented into petty fiefs, the emperor struggles with the Pope, and the Holy Father declares that all those who go to liberate the Holy Land will be freed of their sins. Now is the time for greatness.
If you find it amusing to be the absolute monarch of a particular nation and own everything you can see, this may be the game for you. Inbred courtiers, syphilis-ridden bishops, maimed cripples and possessed, paranoid lunatics will be at your beck and call to do whatever it is that God-Kings do. Be that you want to drown your incapable rival in the bath, or throw your own mum off the castle balcony with a wooden horse, you can do it, provided you don't get excommunicated in the process.
It's a really great game. There is a lot of DLC (21 items!), but most of it is cosmetic. The big ones you will probably want:
Sunset Invasion (Aztecs invade! (Ahistorical))
Legacy of Rome (Byzantine stuff)
Sword of Islam ( lets you play muslim rulers )
The Republic (lets you play a trader)