We have no idea of kingdoms pre-Zeal... for that matter we dont have much about kingdoms post-Zeal and pre-Guardia.... Fact is empires could have came and fell, came and fell.
Another fact about their world, there were magically potent monsters. Our humans had far less competition. Creatures like the Mystics and the Beasts were far stronger than the animals we faced against. You can't kill the Beasts with one hit of Lucca's gun, but considering Lucca has a very advanced gun, I am positive it could take out a tiger or even a mammoth.
Human society actually dates back to 8000 BC... aka the neolithic era, with the creations of permanent settlements (you could make the arguement that it starts at 25,000 BC with the paleolithic era) ... civilization starts at roughly 3100 BC in the city states of sumer...
We have not reached the level of Zeal... There is no way for us yet to make something like the Ocean Palace, or even something as fast as the blackbird and that size. We have no way of mass harnessing the energy at the core of the earth or sun. Truth is we are just more advanced than Guardia 1000 AD. 1000 AD Guardia = I would say 1900 AD US.