How many soldiers could a kingdom realistically muster?

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I have no doubt that the low level troops were treated as cannon fodder (like they are in all armies including modern ones) but the Mongols also hired in chinese seige engineers (since they had no experience of seige warfare) and they were well treated.

Indeed, once Genghis Khan invaded China and sacked Beijing, he would never have done it without the help of the chinese siege engineers. I don't think he had much use for his light troops during the siege, because he starved out the city, and by the time the siege ended, there was barely a person alive in the city. Those that were were killed and the city burnt to the ground. It was chinese writers as well, that started the account of his history, hired by the Great Khan himself.

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