Depends on who the Mongols are and what the invasion window is like; the unified Empire can probably do it. Jochi ulus alone probably cannot (not least becuase it has to preserve strength against other Mongol sub-kingdoms). Unified empire led by Jochikhanids? Not sure, maybe, though potentially not worth it (the amount of encastellation could really be a factor somewhat more than the quality of the castles).
But if you really think Zalesye is better horse country than say, Greater Poland or Lower Saxony, I would ask you to rethink your assumptions. There's nothing inherent to central Europe that makes it unconquerable by the Mongols, other than being really far away from Mongol centres of power.