[stult]

"Not true, as the Chinese cities at the time were much more fortified than Europe. The Mongols were able to plunder the surrounding lands."

That's a bit of an oversimplification. China was much closer to the Mongols' logistical base. By the 1240s, as the Horde pushed into Central Europe, they were experiencing rebellions in Asia that threatened their supply lines. So extended sieges were not feasible.

It's telling that the limit of their incursions pretty closely adheres to the boundary of stone fortification diffusion in Central Europe at that time. Polish and Russian nobles still favored wood fortifications, though Western Europeans had brought back defensive masonry techniques from the Crusades and rapidly developed stone castle infrastructure in their home regions. Stone fortifications increased the time required for a siege and increased the cost in men and material, even with the Mongols' relatively effective siege tactics. The Mongols could not afford to conduct such sieges so far from their logistical base on the steppes, especially not while maintaining control of their often fractious conquests in Central Asia and Russia.

So it's a confluence of factors that limited the Mongol advance, including the relatively challenging terrain of Central European mountains and forests and the increasing frequency of heavily fortified stone-walled fortifications as they pushed farther west.

That said, outside the particular example of the 13th century Mongols, the effectiveness of steppe light cavalry against fortified European cities is at best questionable. The Avars and Hungarians were defeated by western nations once they attempted to push into fortified territory, despite the lack of stone fortifications in 9th and 10th century Europe. And, as you say, it was the incorporation of Chinese siege tactics that permitted the Mongols to succeed against fortified cities, not their prowess as light cavalry archers.

[vkazanov]

Just a note: there was no such state as "Russia" back then. The whole region was a number a big and small duchies left after the fall of the older feudal state, Kievan Rus'.

Later, by the end of the 15-th century, most of them were reunited under the rule of either the Grand Duchy of Lithuania or the Grand Duchy of Vladimir and Moscow (which would, two centuries later, become what we know today as Russian Empire, or Russia).

Anyway, Mongols had no problems crashing both quite a few times between 13-16 centuries. :-) Every state there was in the region back then had to paid the Horde.