General tip : you may have foes, but your only real foe is the Mongol. All the rest you'll crush easily. Mongols are known to have made world conquests - at the expense of experimented players.

Which means that you have to be strong around 1220-1230, and keep strong muslim kingdoms in the east. Crusading in Spain is top notch, crusading in the middle-east, at least before the Mongol problem is solved, is long-term suicide. They will not stop the Horde, they will slow it down enough so that you counter-attack & grab the whole lot.

Which means you have to avoid having too much knights in your armies. The feodal contract is to avoid. It will help against everyone else, but as the Mongols are your only really dangerous foe, and they'll make sheeshkebab from your knights, forget it.

Which means you have to learn guerilla warfare. Mongols can't be beaten on 1 against 1, & if you add more troops, attrition will kill you. So you have to make medium-sized armies, besiege your enemy where he is not, and wait for him dividing its forces. That's long, tough, & you'll probably experience bitter losses even before the first waves of reinforcements. But here is the real challenge. The rest is childplay - use it to get ready for the end-level boss.