The mongols where shockingly good at defeating fortifications see the 13 day siege of Baghdad etc.

https://en.wikipedia.org/wiki/Siege_of_Baghdad_(1258)

They were! The Mongols were a true, global super power, on oar with Alexanders Macedons and ancient Rome.

In the very large set of "this was particularly nasty" things that make up the early middle ages, this one really stands out. I have little doubt that this siege and subsequent sacking effectively set back humanity as a whole - and for the sole and simple reason that the caliph sent a snooty reply back to the Mongols.

The Mongols failed because the Khan died and the army had to retrain and after that the empire fractured. After that the 4 major part all deal with many local issues and also fight each other.

It was four heirs, wasn't it? Charlemagne had two, a split that still shows in Europe to this day.

And I'd hardly say the Mongols failed just because they couldn't hold to western Europe.

> It was four heirs, wasn't it?

The direct heir actually didn't split. It was only later the families split.

> And I'd hardly say the Mongols failed just because they couldn't hold to western Europe.

I mean failed to hold Europe.

I really have to read up on the Mongols, and central asian history in general. That subject is terribly undercoverd in Western history. For somewhat understandable reasons, but still.

>Charlemagne had two, a split that still shows in Europe to this day.

Charlemagne passed the entire empire to Louis the Pious, and Louis the Pious had three heirs that split the empire. See https://en.wikipedia.org/wiki/Treaty_of_Verdun