The politics of feudal rulership permit and even encourage assassinations. This is by far the most interesting part of the game. It's all about finding nations using a form of semi-salic law (which permits daughters to inherit if all male heirs are dead) and killing off said male heirs after marrying the first daughter. The piety and prestige cost for failure is enormous, but the payoff is worth it. Odds are often slim but if the court has a good spymaster appointed, they improve.
The court is another point in the game's favor. Court members come and go as their provinces are brought in and out of the ruler's demesne. Employing the courtier with the best diplomacy value as a chancellor, and the best militarist as a marshal makes all the difference. Combine these with the "guided" rather than controlled research, the different systems of law, controlling the power balance between peasants, burghers, priests and knights in any province - and you have a large amount of depth that's also far enough out of the player's direct control so as not to induce micromanagement or the feeling that this isn't a medieval state.
Wars against Christian nations aren't encouraged since they're so costly in terms of reputation. A bad reputation will lead the vassals to lose loyalty and revolt. Putting these revolts down almost and re-acquiring control of the provinces simply results in a worse reputation. Eventually, the player either has to give up, or personally take control of the rogue counties. Since there is a limit to how many of these provinces can be controlled at a time before tax efficiency is lost, taking back all the lands can mean decades without income while the reputation repairs itself.
Of course, these are the days of fervent Christianity, where the word of God was spread by fire and sword into Muslim lands as the Pope called for Crusade after Crusade. So, beginning in the year 1100, the player finds himself - alone, apparently - under Crusade obligations. While the AI nations have been known to occasionally venture out to reconquer the Holy Land, their efforts are weak, un-coordinated and often disastrous. If you've ever wanted to see the Holy Roman Emperor (AKA King of Germany) as a Muslim, this is the game for you.
There are so many problems with the entire Crusading concept it would take a page to describe each in detail, so we'll settle for a brief list. Crusade obligations force the player to take action by reducing his Piety rating. If Piety goes low, the player is susceptible to excommunication (ie, other Christian nations will fall on you like Bricktop's pigs on a fresh corpse), and your vassals become rebellious. Eventually the pressure becomes great enough that the King's Army and vassals are called to arms, march off, find a weak Pagan or Muslim province and start "converting". Suspension of disbelief flies right out the window as soon as the player realizes he can march through Muslim-occupied Jerusalem as easily as the Muslims can walk through Rome, so long as the factions aren't at war with each other. The very same Pope calling for Crusades against the heathens will let them march from southern Italy to the North through his own state. It's utterly mind-boggling.
Eventually, unless one of the two major Muslim states falls apart into its smaller components, the easy pickings run out. Now as the loss of piety increases over time - meaning that conquests have to become bigger with every passing decade in order to just keep Piety in the positives - the pressure to attack builds. Does your 7-province Kingdom of Scotland dare take on the mighty Fatimid dynasty? Surely their weak, unseaworthy Mediterranean galleys would never be able to sail the dreaded Atlantic, Bay of Biscay, English Channel and North Sea, right? Wrong. Roughly 90% of the time it's possible to smash a few armies, capture a couple of provinces and the Fatimids will figure the war's not worth it and they'll agree to a peace of one kind or another. The other 10% of the time, the AI decides it shall wipe your dynasty and people off the face of the Earth. Its "weak, unseaworthy Mediterranean galleys" will cross the worst seas in the northern hemisphere in order to crush your petty kingdom, and there's nothing you can do about it since allies are completely untrustworthy.
Now, on the off chance your king should conquer Jerusalem, smash the combined arms of the Islamic armies and bring Christianity to the middle east, he shall bask in thousands upon thousands of piety points. So your dynasty is set for good, you shall never have to worry about another Crusade again… and then your king dies. Your new king, if you chose an ecclesiastical education for him, might have 10 piety. At this point, a slightly insane laugh escapes your throat as you click the quit button. The game mechanics are simply too obviously trying to manipulate the player and too tough in doing so. Crusader Kings puts too much emphasis on success - on actually capturing and converting heathen provinces - while not giving any ability to co-ordinate the Crusades with fellow Christian nations.
Historically, only the first three Crusades achieved much, the fourth was actually excommunicated en masse and later sacked Constantinople (and pushed the weakening Byzantine empire permanently into decline, ironically enough permitting a relatively quick conquest by various Muslim Turks). Even these generally unsuccessful ventures were far more organized than what the game permits.