Crusades are "Holy Wars" fought by Catholics against Muslims to take control of the holy land (Jerusalem, Acre, etc.). you can only participate in them if you are of the Catholic religion, not Orthodox. Only the Pope and Catholic factions can call crusades. You cannot participate in a crusade if you are excommunicated. Note however, if you are on a crusade and you get excommunicated your army might desert.
 Crusade Units
Entering a crusade gives the player access to special crusader units and sometimes a "fair in rule" trait. All crusader units have free upkeep until the crusade ends. These units remain after the crusade is finished, but they will then require upkeep. Joining a crusade may allow access to (depending on regional availability):
- Great Cross: A crusading standard somewhat like the Carroccio standard.
- Crusader Knights: Similar to French Noble Knights
- Crusader Sergeants: Crusading Armoured Sergeants
- Unhorsed Knights: Powerful Dismounted Knights
- Note: Knights Templar and Knights Hospitaller become available through their respective guilds, which become available after crusade activities.
 Crusade Info
Once in a while the Pope calls a crusade, and usually calls the first one against a far-off Islamic city. This automatically creates a mission in the player's mission log, but the Pope may also require a specific character to join the crusade as a separate mission. If nobody joins then the crusade ends, and unsuccessful crusades will eventually expire.
Crusades are called on rebel cities, cities of Islam, and excommunicated cities. You can call a crusade if your favour points are high enough. The pope will probably call a crusade on the first city it sees.
The Muslim equivalent is the Jihad.