This may be common knowledge, but I can't recall ever having seen it posted.

Crusades never cooperate with each other. I've seen as many as three Crusades bottled up in Hungary at the same time, all stopped by one Muslim army in Constantinople. None of the Crusades are strong enough to dislodge the Muslims by itself, but any two of them together could.

Likewise, the Hungarians are often allied to at least one of the Crusading factions, but never join in to an attack on Constantinople.

I'm not complaining. It sure makes playing a Muslim faction fun and easy.