What did the crusaders learn from the Muslims?

Europe was going at that time thought so called dark ages, the whole continent was split in to many tiny regions, there central power, regular people had to relay on them selves to protect their communities, there was a lot of injustice, violence and starvation and no hope for better times at all. The society as we know it did not existed outside the towns, country side was not a nice place to live. Small warlords with their own armies was the rulers of this divided continent. Often with ongoing conflicts between them. The Christianity was in turmoil, split between the Greek church of east and Latin west as Urban the second became the pope. This was 400 years after Muslims settled in Jerusalem. So he came with a perfectly prepared plan, how to gather finance and unite people. Because there was no real reason for a war on Islam, created a successfully hate Propaganda against the Muslims describing them as the evil almost unmans, raping women, killing children enemies of god. He promised people wealth in return, but most people joined to sacrifice for their sins. Between 60 and 100 thousand people joined the crusade to Jerusalem. Poor and rich, together with their families willing to cleanse their souls. This is how the 200 yeas of war began.

But, the Muslim culture and science was far more advanced, so was the trade and the army. Islam was divided too but this gave them a reason to unite and become stronger.

The christian church leaders learned few things : how to justify violence and the blooodshed ? And how to finance a war? Simply convincing people that this is a manifest of Lords will and a holy mission, you must sacrifice. God is with you! - a very simple and powerful idea (a better afterlife) that happened to work for centuries, even today.

What did the regular soldier learned in battle? Probably Many christian soldiers was shocked when first seen Constantinopol. There was not a place the size of this city nowhere in whole Western Europe. And this will not be a easy win, when your enemy is a religious fanatic like yourself. By the time they got there, all the horses died, and half of all people too as a cause of starvation.

Western civilization probably wouldn't exist today without learning from Muslims.

philosophy, math, cosmology, navigation, trade, they knew more, and we learned a lot of them, a good example is : the numbers! They brought them from India and we took them and replaced the Greek numbers. This enabled to carry out almost unimaginable complex calculation. There would be no economy, no trade, no banks, no advance