&quot;Huge, flat battlefieds&quot; were a norm everywhere. Medieval battles were ONLY possible on flat surfaces, as the main force of the time - heavy cavalry - requires that.<p>Besides, Mongols completely plundered Rus&#x27; and the Grand Dutchy of Lithuania, and most of those lands were and are forests&#x2F;lakes&#x2F;swamps. For example, they used rivers in winters to travel through the countryside.<p>Also, after conquering China they also had military engineers, and could build assault machines.