A Chinese city must have horses and iron in its Strategic Resource box to build a Rider.
The age of mounted combat has generally been viewed from a European perspective, since it was there that infantry was overthrown and that the greatest and most far-reaching changes in cavalry tactics occurred. But it was by no means an exclusively European phenomenon; indeed, the mounted warrior's tactical supremacy was less complete in western Europe than in the vast Asiatic steppes that bordered China. Evidence from the region suggests that horses were first mounted about 4000 BC, but their role in warfare was trivial until the Han dynasty. The fame of the large Parthian warhorses reached the Chinese imperial court and led the emperor Han Wu Ti to send an expedition westward as far as Fergana to bring back specimens of the new breed in 101 BC. These were mated with the sturdy Mongolian horses, and for the next several centuries the Chinese emperors employed both horse archers and armored horsemen to guard the borders against barbarian incursions and the vital Silk Road caravans from raiders. Ironically, the invading Mongols would eventually turn these against Chinese rulers themselves.