I've specifically stated that steppe people only entered China AFTER the Chinese basically tore themselves up from the inside. When their empires were functioning normally, sorry, but all the steppe people could muster were occasional raiding parties along the border frontiers.
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The Mongols, at the prime of Genghis Khan, took 23 years to finally break through Jin and conquer them, because they've met tremendous resistance in the Chinese cities. Under a normal situation, no steppe leader has enough political power to put their tribe through a war that takes decades. BECAUSE THE MONGOLS WERE AN ANOMALY IN THEIR CONSOLIDATION OF POWER these campaigns were possible -- which I have also mentioned.
You couldn't. That's not how socio-political conditions work.
The fact that steppe people could field so many cavalry, comes from their overall material situation. Their life style that produces such a strong field army, in turn, makes it impossible for them to maintain a consolidated military force for long enough a time to run yearslong campaigns to really conquer anything. Which is why the process of the Mongols coming to power, and forming a tight political control under the power of the Khan was something unprecedented and unseen in the steppes.
Because the Khan held such unusual power, he could assert enough will to keep the army besieging major fortifications despite heavy losses with no progress, for years.
More than a thousand years prior to the Mongols, the Xiongnu were a steppe confederation of comparable power and cohesion. Their "raiding parties" were tens or hundreds of thousands strong and while you could say they were concentrated around frontier areas, the Han Empire at the time was a very large state when speaking geographically, even if considerably smaller than modern China.
For one example:
In the fourteenth year of Emperor Wen’s reign the Shanyu led a force of 140,000 horsemen though the Chaona and Xiao passes, killing Sun Ang, the chief commandment of Beidi province, and carrying off large numbers of peoples and animals. Eventually he rode as far as Penyang, sent a surprise force to break into and burn the Huizhong Palace, and dispatched scouts as far as the Palace of Sweets Springs in Yong [bringing them within eyesight of the capital].
To put this into terms more easily understood by this forum, it would be equivalent to raiding an area larger than entirety of England in 1066 except with more men than William the Conqueror and Harold fielded at the Battle of Hastings, combined. Then doubled. And then doubled again. The Xiongnu went from their the home on the Mongolian Plateau all the way to what the Han Empire itself considered the interior of their lands. And this was not a one-off event, but a persistent threat against the Han Empire for decades.
In other words, when speaking of his ability to create a unified steppe confederation, Ghengis Khan was
unusual but not
exceptional. There were other major steppe confederations of the Mongolian plateau, to say nothing of relatively stable, long-lasting steppe confederations elsewhere on the Eurasian landmass, such as the Turkic conquerors, the Parthians, the Cuman-Kipchaks, the Khazars, and others.
As for Bannerlord, canonically the Khuzaits figured out siege warfare under Urkhun, a few decades prior to the game's timeline. They also have foot archers, spearmen, cities, etc. They are basically half-sedentary already, so why shouldn't they be able to build siege engines? The Vikings, Kievan Rus and Celts weren't exactly known for their siegecraft but nobody complains that Sturgians or Battanians can siege stuff down.