As a Catholic, you can just sit back and laugh as the Muslims get torn apart, get your armies ready, Jerusalem is ripe for the taking!! As a Orthodox, you should be cautious, watch their expansion carefully, if they reach your borders which they most likely won't you could potentially be next in their quest to gobble up the map. To deal with the scourge, you should wall off all acesses to your base with bunkers and fill them with marines and firebats, wait, that's the wrong game, sorry. And as a Catholic Ruler, you can just sit back and laugh your ass off as the Mongols tear apart the Muslims who are probably in civil war, Jerusalem will be much easier to take while the Muslims are busy fighting the Mongols. The Mongols attack nations with a Casus Belli that they have on every realm that is not Mongol, it is basically a subjugation CB, as in most games, I find that the lords that don't die in battle retain a few meagre titles and serve the Mongol opressors, but they eventually get replaced by Mongols, did I mention the Mongols get Unlimited Demense? As Basilieus of the Byzantines, you don't have to worry as much, the Illkhanate usually never reach the Mediterranean and because of their huge threat to the Muslims, you'll find a lot less or at least laughibly weak Jihads comming your way. The next thing you need to know is that these armies are attrition proof, no soldiers will be lost unless in combat, that means you could see a 100k doomstack without a skull near the numbers.

You're going to ask another question, 'They have no provinces, how do they win?' Simple, the game gives them 4 - 6 doomstacks of up to 20k soldiers to loot and pillage as they please. In your first playthrough, you will immediately look to the east, you'll ask, 'I don't see a Mongol realm, where are they?' The Mongols don't start with provinces, they are known as the Illkhanate, they will attack one of the realms at the Southeastern edge of the map, most likely the Seljuks or Khiva. A few months later, you will recieve a second message, the Scourge has arrived!! At around 1216 - 1240 (In my experiences), you will recieve a message detailing the rumours of a Mongolian Khan uniting all the tribes and are heading your way, if you studied your history you will know that these rumours are unfortunately true. If you start from the very beggining at Stamford Bridge, you will get almost 155 years to prepare for the atrocities that will soon arrive from the East. And as an end to this introduction, I will quote the words of Captain Kirk, Star Trek Wrath of Khan 'KHAN!!!!!!'. There are many obstacles for the Mongols, but in this page, you will understand that no obstacle is too large for the Horde.

As a ruler or vassal of the West, it is your duty to protect the realm from this seemingly neverending horde of Steppe Wolves. In almost every game of Crusader Kings II, there will come a time when an army of vicious pillagers known as the Mongols will come to seek conquest and riches in the West.

Crusader kings 2 provinces