In a way, the crusades both the strengthened and sounded the death knell for the feudal system in Europe.
They solved one problem of too many knights in Europe wandering around killing people and disrupting life in general. The Crusades siphoned off this force and put them to work killing non-Christians. And pretty much every generation or so, there was another Crusade, another call for excess Christian thugs to join the hooligans in the east.
But the Crusades did two things which ultimately undid the feudal system. First, they provided a trade route and a taste of the exotic things of the East. Until you tasted pepper, you had no idea your food was so bland. The Crusades opened the door for the import/export of commodities, that is, beyond what could be made locally. That is, once you start exporting your labor, including your excess thug labor, well, now you get things moving back and forth that were made elsewhere and the rich develop a taste for the finer things.
Second, once people start going to the Holy Land you've got to start funding the process. Travel expenses and costs on Crusade and treasures looted from Saracens and even Constantinople are all money transactions. And this is way before the first ATM. Travelers checks don't work so well in the highlands of Syria, even American Express. Where there's a need, somebody will resolve it. So the great orders of knights, the Templars and the Hospitalers, in spite of their original purpose, they become bankers. They will take a mortgage on your home and give you a line of credit which can be exchanged for money in the Holy Land. Pretty soon, these orders are moving a lot of money and become the first international bankers. Once that function gets started certain people know they are richer or poorer than other people and this starts squabbles everywhere and kings can borrow money and eventually international banking is here to stay. Oh, the names on the doors have changed, but not the function. This really kills feudalism; no longer are people strictly tied to one spot, they can now work elsewhere and make money, elsewhere.
