The author has
Hell of a weird idea about what
counterweight trebuchets looked like, just sayin'.
Probably also worth noting that the cracks in the Empire started showing long before the succession crisis between Kublai and Ariq Böke (and the
near-simultaneous war between Berke's Golden Horde and Hülegü's Ilkhanate in the Near East) - in fact already during Genghis's lifetime. Notably during the 1221
siege of Urgench, in Khwarezmia, there was a serious falling out between Genghis's eldest son Jochi and both his father and brothers; afterwards when Genghis and the others returned to continue the interrupted conquest of northern China Jochi basically took his forces and buggered off to sulk in northern Central Asia, taking no further part in the wars during his life. His son Batu later led the invasion of Europe - and in
the wake of the Battle of Mohi (Hungary, 1241) fell into rancorous dispute with his nephews Güyük (eldest son of Ögedei) and Büri (grandson of Chagatai)...
That made things pretty awkward when Güyük became the Great Khan in 1246.
De facto Batu's domain, the future Golden Horde, was already essentially autonomous from the rest of the Empire and routinely just did whatever the fuck it wanted. Nomad empires in general were ever notoriously unstable and prone to fragmenting in short order and the enormous physical size of the Mongol one, combined with the unprecedented real power the regional princes had over their domains (previous nomadic rulers had tended to have far looser hold over the component tribes of their realms) and thusly ability to marshal forces for bids of autonomy or outright usurpation, only exacerbated these built-in tendencies. No surprise then that within half-century of its founder's death it was already united in name only and completely came apart at the seams within the next hundred years.