This ancient metropolis is situated on a lava field (a rugged, arid expanse of solidified lava), but its inhabitants transformed it into residential units surrounded by perimeter walls. We know of at least 2,700 in the southern part—the most studied—and it is estimated that at the height of its occupation, it had around 7,500 housing units where just over 90,000 people lived. The presence of 27 or more ball courts gave it unusual significance. It reached its cultural peak between 350 B.C. and 550 A.D. After 600 A.D., its population grew significantly, and during that period—until 900—it was the largest and most important city in the Central Altiplano.