&gt;  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.<p>Just noting that these claims are highly speculative and not widely accepted by historians.<p>&gt; European heavy cavalry was not useless against mongols...<p>Agreed. The mongol skirmishers were not the superhero units some people make them out to be but otoh they were experts at inducing their opponents into favorable engagements.<p>&gt; 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>This is basically correct but I&#x27;m skeptical the larger conclusion is proper. The Hungarians learned something from the first Mongol invasion that had success in the second. Some people&#x27;s takeaway from this is that stone castles were some kind trump card against the mongols generally but given experience in China and elsewhere it&#x27;s more likely just a weakness of that specific invading army. The mongols showed up in Russia&#x2F;Eastern Europe with Chinese siege technology which was enough to beat most everyone they faced ... until it wasn&#x27;t (2nd invasion of hungary, etc.).  They ran into the same problem in China, personified by the Siege of Xiangyang but over years the mongols learned lessons too and increased their siege technology with Arab trebuchets and then every fortified city they faced afterwords in China fell like sand castles. Presumably the same pattern would have repeated in an extended invasion of Europe.<p><a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Battle_of_Xiangyang#Aftermath" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Battle_of_Xiangyang#Aftermath</a><p>&gt; 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.<p>They used all sorts of troops and tactics including lots of Mongolians.<p>&gt; 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>I find this unconvincing, letting the mongols take you apart a small kingdom&#x2F;castle&#x2F;city state at a time was not a successful strategy anywhere as even if you had equal or superior troops the mongols could bring overwhelming numbers to bear on you. This is basically what happened to Russia and most of central Asia. Also Song China had tons of fortified outposts.