Simply the best grand strategy game ever made. Period.
The game takes place in real-time, i.e. no turns. You can pause the game at any time and choose the speed through which the game progresses. This allows plenty of flexibility, depending on the amount of planning and decisions you need to make.
Crusader Kings 2 is a sophisticated game, and it takes some time to realize the arsenal of actions you may be able to take. However, you have no goals in game and no objectives you need to fulfil in order to satisfy some pre-written mission script. You choose to do whatever you wish to do, and behave however you wish to behave.
Your first action in the game is to choose a ruler to control. Depending on the DLCs (downloadable content) you possess – you can choose between a Christian ruler, a Muslim ruler (the Sword of Islam DLC), and a Pagan ruler (the Old Gods DLC). You can also choose between monarchies or republics (The Republic DLC). Theocracies, such as the papacy, are not (yet?) playable. The game can also be set to start at any date between the start date (depending on the DLC) and some hundred years before the ending of the game (14th century-ish) and each date is historically accurate.
The game revolves around the point of view of your character. Controlling, for instance, the Kingdom of England essentially means you're in control of the King of England. Unlike other strategy games; you're not controlling states or empires, but rulers who in turn hold certain titles and have vassals. You may wonder what happens after your character dies. In that situation, you resume control of the heir of your character; this is normally a son or a daughter, but it can be anyone from your dynasty, depending on the laws of inheritance (these can also be legislated by you, with certain limits).
Each character, including your own, has certain characteristics. The most important of them are traits which govern, among other things, your character's fondness and skilfulness. If your character is kind, just and charitable, his or hers vassals will like him or her which would in turn keep rebellion at bay and land you more troops and resources. If your character is a gluttonous, arbitrary wroth, everyone around your character will hate him or her and that will in turn invite trouble. Not all traits however have solely negative or positive impacts. The "ambitious" trait, for example, vastly increases a character's traits, making it more productive, but also leads to tensions between that character and his or hers liege lord. As you'll find from experience, the game revolves around many trade-offs. There are many solutions to many problems, where each solution may introduce new problems.
Combat in the game is macro-managed. Your responsibility is mostly to raise, unite, select leaders and send army forces to battles and sieges. The game behaves realistically in the manner that, most often than not, larger armies win battles and vastly. You can't pull off a situation where 300 men defeat 2000 – you would lose every time, even if you have the best technology and the best leaders against the worst technology with no leaders. In that sense, technology, leaders and terrain perhaps at best give you a 1:2 edge, and this is a very rare situation. There are still, however, good reasons not to put all your eggs in one basket and the most notable of them is a province's ability to support your troops. If you have too many troops in the wrong province, they'll die from hunger and soon enough you'll find your glorious 15k army turning into a rotting 4k loosely associated band.
Each religion in the game has its own rules that control your relationship with other realms. Being a Christian means you cannot attack a Christian neighbour, unless you have a claim for their land or some other cause for war (such as a right to invade given by the head of religion – e.g. the Pope). However, should you border a non-Christian (or a heretic), you would be able to go on a holy war against them and conquer their land. In that sense, religions mitigate the effect of expansionism and largely prevent wars within the religion. In addition to religions, there are also cultures, which also have an effect on the relationship between characters and controlled provinces, as well as other characters.
Most of the time, you're not going to take part in a war, but you would still have to advance your machinations in a certain way. You may want to give yourself reasons to go on a war on a neighbour by fabricating a claim on his or hers title by sending in your chancellor. You may marry your relatives to prominent rulers in order to forge alliances and maybe even create a situation where offspring coming out of this wedlock would be members of your dynasty. You may wish to stabilise your realm by arranging and creating new vassals, or perhaps even by plotting to murder political rivals - these come in the form of dangerous factions within your realm who wish to undermine your authority. Sometimes you may find yourself in an affluent position to build more buildings and improve the stature of your holdings so that they provide more troops and taxes and would be better suited to defend in a siege. There are innumerable amounts of things you can do, however you're never obligated to use all of them.
The world and characters you are interacting with are not stupid. You may for example have enough forces to take on a neighbour's title, so you declare a war and maybe even emerge victorious several times. However, you just forgot the fact the Pope hates your guts for being a cynical, hedonist, lustful bastard (I mean literally a bastard, not a curse word) and your enemy just asked the Pope to excommunicate you. Before you notice, many of your neighbours declare war to depose you and soon enough you lose the campaign. Ok, so maybe the Pope doesn't hate you, but you have a few powerful and ambitious vassals who wish to become independent… they just waited for you to expand half of your army so they can start a rebellion – taking the remaining half of your army with them. Now you're with a quarter of your original army, fighting on two fronts, and soon enough you either plot your way out of it, raise mercenaries to help you out, use diplomacy to attain peace or, again, lose. The bottom line is that there are characters out there, besides you, and they have their wants and wills as well. This is why you have to take a spectrum of considerations into mind before you commit dangerous actions, and take the AI controlled characters very seriously.
All in all, it is very difficult to further describe such an immersive and elaborate game. I would like to point out the fact that, even after a year of its release, the game is in full-scale development. Not in order to fix bugs, but rather to add more features and possible actions. When you've entirely finished an amazing 586 years campaign (which should take you at least three weeks if you play full time), you can export your game to Europa Universalis 4 (if you have it), which is a similar, but vastly unique and different game that takes place in the beginning of the Renaissance until the end of the Imperial age (early 19th century).
Crusader Kings 2 features many DLCs, many of them add graphical and audible content. However, the most important ones affect gameplay, and are:
1. The Old Gods – as I've mentioned earlier, allows an earlier start date and the ability to play as a Pagan ruler. It also involves Viking-like mechanics of raiding and looting.
2. The Republic – allows the player to control a medieval trade republic that has trade ports and a different style of internal politics.
3. The Sword of Islam – allows the player to control Muslim rulers.
4. Legacy of Rome – allows the player to re-create the Roman Empire. This DLC also introduces the ability to recruit standing armies, known as retinues, which is a very important addition in my opinion.
5. Ruler Designer – allows the player to replace the historical rulers of any realm with his or her own customized ruler.
If you intend to buy the game, I strongly advise you to get these five DLCs as well, as they add great content and re-playability.
In conclusion, this IS the game if you fantasize about reliving the middle ages as a noble ruler. Wage war, conduct diplomacy, build and maintain holdings and plot your way into a greater, richer, better, more pious realm. Do that, or see it all turn into disarray if you make the wrong decisions.