I read a long discussion about this on Quora. What I could gather from that was there were far more castles in Europe than China. Also Europe has too little plains to be able to support a mongol army very long. They each had something like 5 horses. There isn&#x27;t enough grass for a large mongol army like that. One the debaters claimed e.g. that European castles were very well supplied for long sieges and would run out of food much later than an occupying mongol force which would be vulnerable to counter attacks.<p>European heavy cavalry was not useless against mongols. It is just that they were seldom effective when mongols could just run away from the battle field. Easy to do when they were much lighter armoured. But if Mongols maintained a siege they had to stay in position which would have made them vulnerable to European nights which were probably superior to mongol soldiers if they could not perform hit and run tactics.<p>Apparently the mongols beat the Hungarians initially but when they came back and the Hungarians had built lots of castles, they were soundly beaten. The sieges dragged on and they would get harassed by Hungarian forces which could always retreat back to castles while mongols starved.<p>My understanding is that the Mongols managed to conquer the more hilly southern China by using conscripted Chinese soldiers. So they were not really using standard Mongolian tactics there. Duplicating this in Europe would have been difficult because unlike China Europe did not have the same kind of central government which would have allow the conquest of a few cities to gain huge tracts of land and extra manpower. They would have had to conquer huge numbers of castles which would have dragged out in time.<p>In fact Machiavelli talks about this in the Prince. The Ottomans, Roman Empire, China etc were far more centrally controlled which made them one strong unit but once you conquered the central city of government you would control the whole country. The Feudalistic European countries however might not as easily marshal huge combined forces but they were extremely difficult to conquer because you had to conquer every little vassal state one by one. Considering that every one had a castle this was no easy task.<p>One should not completely discount the effectiveness of European military. During the crusades, the muslim forces would usually use Mongol like tactics to wear them out. But whenever the crusaders managed to corner a muslim army they crushed it with their heavily armoured knights.