The lumbering Heavy Chariot may look unwieldy, but the poor soldiers crushed under its wheels don't seem to think so. Not only is the chariot the strongest land-based melee unit in the Ancient Era, but it also may move farther than any other unit on open terrain, and it doesn't require any resources!
The Heavy Chariot is in fact a proto-cavalry, unlike the Chariot Archer in Civilization V (which was a ranged unit). But even when used as a regular melee unit it will crush its opponents, thanks to its superior Combat Strength. Its only nemesis is the Spearman, which has a special bonus versus cavalry, bringing its total Combat Strength to 35 against the chariot! So, stay away from any army with lines of Spearmen.
Ox carts – sort of proto-chariots – were first built by early Indo-European peoples in Mesopotamia around 3000 BC. Not long after, c. 2500, horses were domesticated on the Eurasian steppes, a practice that quickly spread. And not long after that, horses got hitched (usually side-by-side) to the carts, which had been made lighter – often just a platform on two wheels with a low railing – and more maneuverable. Besides practical uses for chariots in transport and racing, chariots were very useful in war, giving ancient armies a mobile arm of their military. The Hittites were the first master charioteers in warfare, manned by three men in nimble chariots drawn by two horses. The Egyptians ran roughshod over their enemies using a similar contraption. The Persians were the first to build heavy chariots, with a four-horse team and sometimes scythes on the wheels, which proved really effective in smashing through enemy infantry lines.