Teotihuacan, the mysterious metropolis of ancient Mexico, seems to have provided a blueprint for Aztec city builders a millennia later.

A new re-analysis of the urban planning and layout of Teotihuacan is published in the latest Open Archaeology by Arizona State University archaeologist Michael E. Smith.

“The designers of the city created a series of innovations,” writes Smith. “In some cases these took the form of avoiding standard Mesoamerican urban traits (such as ballcourts or royal palaces), and in other cases these innovations were new features of urban layout (such as the use of a central avenue and apartment compounds as residences).

“By the time the city reached its maximal size, it was an utterly uniquely designed city within Mesoamerica,” he adds.

The city, which was home to as many as 100,000 people at any one time during its heyday from the 1st Century B.C. to the 7th Century A.D., was built at right angles, according to Smith.

The metropolis was successful for centuries, though the reason for its ultimate decline and abandonment remain a mystery.

Other Mesoamerican builders, including the Maya and Olmec, seemingly eschewed the Teotihuacan model in centuries after the city’s fall. Instead, they built around centers with focal points of temples, a palace, a plaza, and ballcourts.

But it was the Aztecs, who were building their capital of Tenochtitlan some 40 miles away from the ruins of Teotihuacan, who took the torch of the urban planning concepts.

They too built on right angles, and with a scale for a population that would grow to become awe-inspiring to the conquering conquistadors.

Differences were present, however. Teotihuacan boasted many apartment dwellings, while Tenochtitlan did not, for example.

“Teotihuacan stood alone as the only city using a new and very different set of planning principles, and its apartment compound represented a unique form of urban resident not just in Mesoamerica but in world urban history,” said Smith, in a statement.

Previous researchers have hypothesized that much of the modern urban planning was prefigured by Teotihuacan. But work continues at the site, including the work of David Carballo of Boston University, who talked about his work with Laboratory Equipment in 2015. Teotihuacan continues to pose riddles and offer surprises to investigators at the site. Also in 2015, an archaeological team digging for a royal tomb under one of the large pyramids in the city found a large amount of liquid mercury stockpiled there.