It's (debatably) rooted in [url=https://en.wikipedia.org/wiki/Seven_Cities_of_Gold_%28myth%29]history[/url]. Even in real life, fabulous cities like Tenochitlan and even Teotihuacan came to exist in the region. Given a more civilized New World - who's to say a state of extreme opulence like Cibola or Pais de la canela couldn't have existed? As long as you aren't building guns or plate armor, I say go for it.

Besides, it'd be good for my nation's plotline.