Final answer:
Tenochtitlán was the capital of the Aztec empire, built on an island in Lake Texcoco, and the site is now within modern Mexico City.
Step-by-step explanation:
The ancient city that served as the capital of the Aztec empire and is now located at the site of modern Mexico City is Tenochtitlán. Renowned for its architectural complexity and stature, Tenochtitlán was built on an island in Lake Texcoco and was the center of the Aztec civilization.
Following the conquest by Hernán Cortés in 1521, the Spanish destroyed the Aztec capital and built Mexico City over its ruins. Today, one can visit the remnants at the Templo Mayor, revealing insights into the once-majestic city and its sophisticated urban planning.