Politics during the medieval period were quite different from the way they're often depicted in popular culture. The king, for example, wasn't usually an iron-fisted dictator laying down all the laws. Agnatic primogeniture - the system in which the oldest son inherits the title - was pretty popular but far from being a universal, unbreakable rule. Due to feudal politics, borders of the states were not set in stone and often changed on inheritance while the laws within a single kingdom could vary significantly from city to city and from county to county. It's something quite difficult to imagine due to how complicated it could get while also being nothing like modern politics - thus, all the intricacies are often omitted in fiction in favor of portraying something more akin to either absolute monarchy or modern parliamentary monarchy.
Crusader Kings by Paradox does things differently. The game, like many others by the developer, goes for historical accuracy and throws all the complex political stuff right into the center. It's a game where success requires you to learn what is scutage, who inherits what under Salic gavelkind inheritance law and which approach to the relationship between Church and secular rulers will not lead to the pope excommunicating you. Crusader Kings focuses on the parts of the middle ages many other strategy games omitted - from Civilization to Age of Empires 2 - and it does some of it well enough that you could actually learn a thing or two from it.
Due to the nature of feudal politics, the player doesn't really control a country. Instead, he assumes the roles of successive members of a chosen dynasty as they inherit titles from their deceased predecessors. Every Christian count, duke and king is playable - but wars, assassinations, title claimants, usurpers, rebellions and inheritance crises mean that the powerful king might be left with only a single province while one of the counts or dukes assumes the throne. It's a great idea - even losing an important war doesn't have to mean game over as your descendants might amass wealth and power to return to the throne.
Of course, those descendants have to be capable leaders themselves. Each character has a set of personal stats and traits which determine how good they are at certain actions (e.g. commanding the troops or assassinating other characters), which is affected by a slightly buggy genetics system (removed in the Deus Vult expansion and replaced with event chains related to education of a child) so it might be a good idea to look at the stats and traits of your potential wife. Of course, if you can't breed or educate your heir well enough for him to be a worthy successor, you can just kill him and get a new one. It's the sequel that's known for many opportunities to be an evil, calculating and ruthless ruler, but the trend already started here.
But evil is not without repercussions. You also need to take care of your piety, prestige and reputation stats, which determine the character's popularity with different groups of people, while also keeping a look out on the loyalty of the individual vassals - they might be lower rank than you but they're rulers too and if they dislike you, they will not aid you in your wars and might even rebel against you. While most strategy games focus on fighting against external threats, Crusader Kings makes you fear your own people as much as you fear the neighbour's armies marching towards your borders.
While all this sounds like a recipe for a great game, Crusader Kings has quite a few issues which make the game much less fun to play than it could be. The most immediate problem is the rather clunky and unintuitive interface - the game was based on the Europa Universalis II engine, which simply wasn't meant for this type of gameplay. While managing and controlling your armies is fine, the micromanagement of your vassals, neighbours, enemies and family members becomes quite tedious - and in a game based so heavily on interpersonal politics, this is unforgivable. Deus Vult fixes a few of those problems by giving you notice about vassals with low loyalty, vacant council positions and other pressing issues, but it does little to simplify even the most common actions.
The game also feels almost unfinished in several respects. There are random and semi-random events and event chains in the game but they're all short and simple: A thing happens, so how does your character react? Each character is represented by a portrait, but there's quite a small number of them and while it changes with the character's position on the feudal ladder, it doesn't take some rather simple things into account and you always end up with popes dressed like kings just because the papacy is considered a kingdom. The music is similarly limited and you'll get tired of hearing the same tracks over and over again pretty quickly. Crusader Kings is also quite a buggy game and while many of the bugs were fixed in Deus Vult, it's important to remember that the expansion was released three years after the vanilla game.
All those issues could be forgiveable if the game beneath the surface was solid. Unfortunately, it isn't - and for reasons that would have become obvious with thorough playtesting. There's not much to do in Crusader Kings. You can build improvements, but the benefits they provide are often useless as peacetime gameplay is generally easy, so the main objective is always making your kingdom bigger through clever arrangement of marriages and assassinations or warfare. Assassinations aren't very reliable (unless you breed a high-intrigue king and a perfect spymaster), so you're probably going to go to war. Getting a casus belli against Christians (and more often than not you're going to be surrounded by Christians) takes quite a long time, while the warfare itself is slow and wastes all your resources. There is no standing army and having troops raised is expensive, taxes and laws that are good for your armies make vassals less loyal and fighting Christians lowers your reputation. Therefore it's best to keep it as short as possible and recover losses afterwards. As a result, you're going to spend most of the game waiting and the rest desperately micromanaging laws and taxes so that you can win without having everyone rebel against you.
Crusader Kings cannot rightly be called a good game. It has too many problems, it was buggy at release (even for Paradox standards) and it seems that many ideas behind it weren't fully realized. It's an ambitious title for sure - but that doesn't mean it's fun to play. While some of the game's issues have been fixed in Deus Vult, it was not until the sequel that the true potential of Crusader Kings was unleashed. In retrospect, the first game was little more than a prelude to the greatness of Crusader Kings II.
Any short description of vanilla Crusader Kings II is going to sound almost exactly like the first Crusader Kings: once again, the player controls a medieval dynasty. Once again, there are no clearly defined goals. Once again, there's a big focus on interpersonal relationships and keeping your vassals happy. Marriages, assassinations, laws, councils and inheritance crises are back as well. The biggest difference here is that Crusader Kings II actually does most of those things right, fixing almost all the problems present in the first game while also adding not only a lot more content but also a lot more personality.
The sequel's interface is easy to use, intuitive and customizable. The pacing is also much better due to a more involved assassination system (it now includes the need to find supporters and a chain of event describing how the plan was executed), significantly larger number of events, with some of them - like feasts, hunts and tournaments - triggered by player's decision. The laws often require vassals' approval, there are expanded personal diplomacy options and the ambitions system in which players can set goals for their characters, so there's a lot more to do during peacetime. It's rarely necessary to fast-forward through boring parts of the game as there's quite a lot to do now and it takes a lot of time before it gets repetitive.
Probably the most important change from the first Crusader Kings is the replacement of general reputation and vassal loyalty with an opinion mechanic. Every single character in the game has an opinion about all the other characters ranging from -100 to 100. Like loyalty and reputation, those are influenced by your political decisions (discovered assassinations, strict laws, raising troops for too long etc.) but this is not the end of it. Opinion changes with events (and this change affects only those present) and decisions. It's also heavily dependent on culture, religion (people are often wary of foreigners and infidels) and traits - people tend to dislike you if your traits are opposite of theirs (e.g. lustful vs chaste), they can have an attraction towards different ones and the effects can be situational (eg., you can get along with an ambitious ruler but nothing good can come out of having an ambitious vassal). Almost everything can influence someone's opinion - and you'll never satisfy everybody.
Another important change is the writing of the events. Instead of a few short sentences, the player is now presented with a longer, well-written description of the event. The event chains can also be pretty fun. For example, after a character dies, his very young child may become the king and the appointed regent decides to educate the new ruler. Soon, the regent may start forcing the child to perform some rather dangerous tasks that could lead to his untimely death. As the young king survives every single one of them, the regent decides to take a more hands-on approach to getting rid of the child and tries stabbing him with a knife, but his assassination attempt is thwarted by the guards who throw him into the dungeon. There are many cool events like that in Crusader Kings II and some of them are pretty hard to find on purpose, but the game is such a time sink you'll probably happen upon many of them just by playing it the normal way.
While all the improvements over the first Crusader Kings (which also include expanded laws and technologies) would make the sequel an undoubtedly great game, it's the expansion packs that make Crusader Kings II really shine. All the major DLCs add huge amounts of content to the game and turn it from a strategy game with a specific focus into an amazingly detailed medieval sandbox. Instead of a monarchy, you can be part of a republic. Instead of being a Christian, you can worship various gods from Odin to Allah. And it's not just cosmetic changes - all the government types and religions have completely different mechanics, while many cultures are given unique laws (tanistry, cognatic succession) or a special casus belli (tribal invasion). The expansions also allow the players to start the game as early as in the 8th century (as opposed to the 11th in the vanilla game), introduce alternate history elements and generally play as almost anyone who ever had a piece of land (with Wikipedia links in the profile of every historical character) in medieval Europe, North Africa, Middle East and India.
There's a lot of detail put into all those unique mechanics, too - non-Christian religions get everything from different views on marriage (e.g. Muslims can get up to four wives) and their own festivals to things like human sacrifice (available to Norse pagans, Kali worshippers and - if you don't mind alternative history scenarios - Aztecs). Each religion also offers different challenges: pagans must reform their religion so that it becomes organized enough to survive, islamic dynasties have to control decadence so that they don't enrage zealous desert tribes and the inhabitants of Indian subcontinent must choose between pacifistic Jainism, militaristic Hinduism and learning-oriented Buddhism with their various sects while keeping the caste system in mind. If those are not difficult enough, one can also attempt to bring back Zoroastrianism, Judaism or worshippers of solar god Zun to their former glory. Unfortunately, not all religions were made equal and while there's been a lot of detail put into Norse, Tengri and Hinduist faiths, the same cannot be said about those who believe in Slavic, Baltic or African gods.
The great variety of playable characters, focus on intrigue and personal life and a great number of well-written events mean that, with a little imagination, Crusader Kings II can become one of the best games for emergent storytelling. Every playthrough of the game will be filled with conspiracies and rebellions, great heroes and evil tyrants, love, war, betrayal and death. Player characters can become anything from scholars and mystics to rapists and murderers (or maybe all of the above at the same time) and the games has a tendency to force you to make difficult choices - killing your son or letting the kingdom be inherited by an incapable ruler, marrying your lover or someone who'll give you a good alliance, getting into debt or raising the taxes and risking a violent revolt. Fans of A Song of Ice and Fire / Game of Thrones will feel like at home (there are even fanmade mods to replace Europe with Westeros) and so will alternate history enthusiasts.
Crusader Kings II is also a great game for those who love depth and complexity. Every religion plays differently, each aspect of the game has its own mechanics that need to be mastered and the game offers many optional challenges like 'ironman mode' (the only way to actually get achievements) in which the game prevents savescumming by limiting players to a single savefile and saving automatically. It might be quite tough for the beginners, but still less so than many other Paradox games (e.g. Hearts of Iron) and the difficulty itself depends heavily on starting date and chosen dynasty. The patience needed to learn the game will be rewarded, as Crusader Kings II is known for having players immerse themselves in the game and playing it for hundreds of hours.
Crusader Kings II is a nice-looking game. While you will spend most of the time looking at maps (especially the realm map which shows the different kingdoms), there's a pretty cool option of switching to a terrain view with mountains, forests, rivers and towns when zoomed in. Loading screens are decorated with paintings depicting various scenes from medieval life (crowning of the kings, pilgrimages, crusades) and there's a nice touch of each religion having slightly different interface icons. There are also decent unit models and dynamically changing character portraits, but those have to be bought separately for each culture unless you want to look at the same few pictures over and over again.
Music in the game is not bad either and it evokes the medieval atmosphere, although most of the tracks don't really stand out. It might also get a bit repetitive after a while - this can be remedied by getting DLCs with additional songs but once again, you're expected to pay for each pack separately. There aren't many sound effects in Crusader Kings II and those that are present aren't especially interesting - there are clicks when using any of the game's many menu screens, there's the ringing of coins when you receive taxes and the sound of combat when viewing the progress of a battle - and that's all there is.
Crusader Kings II is a great game. While not without its faults (some religions could be fleshed out a bit more, the "Way of Life" expansion isn't that great and Paradox really wants to nickel and dime people with all the unit, portrait and music packs), the deep and complex gameplay combined with good writing, great storytelling capabilities and a lot of freedom. It's also probably the most historically accurate medieval game ever produced - covering anything from law and military to culture and religion. It's also one of the games that are simply too complex to describe in their entirety in a single article - merchant republics, tribes, religious wars, mercenaries and military orders are all important parts of the game that haven't been mentioned here. There's a lot to like about this game - provided you have time and patience to learn its intricacies.