The ancient civilization that built the city of Teotihuacan is the Teotihuacan civilization, which thrived in Mesoamerica around 100 BCE to 750 CE. Located near modern-day Mexico City, Teotihuacan was one of the largest cities in the ancient world, characterized by its impressive architectural feats, such as the Pyramid of the Sun and the Pyramid of the Moon. This civilization was marked by its urban planning, complex society, and extensive trade network. The reasons for Teotihuacan's decline remain debated, but its cultural influence on later civilizations, including the Aztecs, is undisputed.