El Tajín, Prehispanic City is a site with great significance for Mesoamerican archaeology because it is one of the best preserved and most thoroughly excavated examples of a pre-Hispanic town from the Epiclassic and early Post Classic period, the time between the fall of Teotihuacan and the rise of the Aztec empire.

Source : UNESCO

It's estimated to have been at its peak between 800 to 1200 A.D, with roughly 20,000 inhabitants.

The site is located in Mexico in the highlands of the municipality of Papantla in modern-day Veracruz, not far from the city of Poza Rica, which lies northwest of the port and city of Veracruz.

Source : Wikipedia

It's not known who built the city :

Some argue in favor of the Totonacs and the Xapaneca; however, there is a significant amount of evidence that the area was populated by the Huastec at the time the settlement.

[link as above]

Or, why it was eventually abandoned.