The TIL in question. It links to this article. The second page of the article has the sources. The source for this fact seems to be the book Genghis Khan: Conqueror of the World which has bad-ish reviews (although not from historians). I also checked wikipedia just in case but while its mentioned there there's no source for it.
Can someone please tell me if its true?
P.S. Essentially my question is asking if the source given in the article is a good one. How do you guys judge a good source from a bad one?
The story actually comes from Juvayni's The History of The World Conqueror, who was a contemporary of the mongol invasions and later became an important official in the Il-Khanate administration. Juvayni is great because we have so few contemporary sources of the mongol conquests but he does have a flair for the dramatic and inflating numbers. Juvayni claims they went full Carthage on the city, killing every man woman, child, dog and cat, along with levelling every house and palace.
While Nishapur would be one of the most populated cities in the world at the time, it's highly unlikely they had a population that high. The 1.75 million may have come from combining multiple cities in the region but even then i can't find a source for it.
1.7 million in a city? Maybe more of a follow up question, but were there even cities that big in the 12th-13th century?