Well, I think they are not done, but here's how they work so far.
A crusade is, in many ways, like a regular war. But it has some uniqueness.
Step 1) Pope calls a crusade against a specific county (say, Jerusalem or Acre).
Step 2) All Roman Catholics get a Crusade CB against the owner. Note that the Crusade CB is slightly different than the Holy War CB which you have at all times against non-Christians. The Crusade CB adds extra piety if you win.
Step 3) Rulers who think they're cool enough DOW the target. All wars are independent (meaning that if King A and King B are on crusade, their wars are separate).
Step 4) Your character arrives in the Holy Land (or general vicinity of the target). You get a trait "Crusader" adding +2 to martial and boosting your relations with clergy and other crusaders. Also, since you are in the area of the Crusade Target, the game recognizes this. Bishops and the Pope like you more. You are also immunized from "Hey, dummy, go on crusade" events.
Step 5) Fight battles, win sieges, kick butt.
Step 6) Force the enemy to give you the crusade target in the peace treaty.
Step 7) Profit! (Which currently includes a pile of prestige and piety)
So, I wonder if Constantinople is considered the general vicinity when the crusading target is Egypt...
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