I must also say I don't really see the difference between an "ethnic" Mongol soldier in the Mongol military system and a, for off-the-top-of-head example, Iranian Turkish soldier in the Mongol military system. The two - steppe nomad warriors organized to fight under the same methods - ought to be about the exact same thing in just about all practical respects.

European castles were privately built affairs. They did not follow geographical or political (i.e. border) lines, but simply what the local ruler saw as the best location. A direct result of this was that European fortifications ran pretty haphazardly through the landscape, making it easy to bypass each;
And that "best location" just so tended to happen to be the best location for defying incursions and raiders. European feudalism to a large degree developed to defeat mobile raiders (the Moors, Hungarian-Magyars and Vikings originally), and its later internal wars primarily consisted of laying waste each others' holdings (as taking fortified places tended to be a bit of a challenge); the fortress networks were quite good indeed for checking enemy movements and supporting each other as needed. You could say that "area control" in more ways than one was what they were all about.

The only "haphazard" about them was where a lord was able to build them in the face of his competitors and within the limits of his territory; they were certainly expensive and important enough that they weren't just scattered about randomly.

Forests? Come on. Did that stop them in Russia -- a nation which still has much deeper and larger forests than Western Europe, even today?
Uh... you know, I don't think the Mongols ever ventured too far into the coniferous forest belt, save for raids. No nomads who'd been inhabiting the region since before they learned to ride horses ever tended to, either.

Due to the simple fact they didn't need to, nor want to because especially after they went on horseback they'd have been at quite the disadvantage there.

The Mongols, like all the nomad empires before them, took over the steppe part of Russia. The forested bit was largely left alone, or in any case not actually conquered and at best adminstered by local vassal lords under the threat of punitive expeditions.

Actually when you look at it, China - which was half plains anyway, and could be conquered relatively easily by taking control of the strategic nerve centers - and Korea were about the exact only places where Mongol dominion actually went well past the limits of the Great Eurasian Steppe Belt. They kind of seemed to hit a wall (à la Vietnam, Java, Japan and Egypt/Asia Minor for some), or just give up and leave (à la Hungary and Poland), almost everywhere else.

I'd say the pattern is a bit too pronounced to be coincidential.