The Aztec capital Tenochtitlan was larger than any city in Europe when Cortés arrived

When Hernán Cortés and his men arrived in 1519, Tenochtitlan — built on a lake island and connected to the shore by causeways — had a population of roughly 200,000 to 300,000, making it larger than London, Paris, or Seville at the time. Its market at Tlatelolco was described by Spanish soldiers as bigger and better organised than anything in Spain. The city had a complex sewage system, botanical gardens, and an aqueduct. Cortés destroyed it almost entirely by 1521; Mexico City was built on the ruins.

Source: Restall, M., When Montezuma Met Cortés (Ecco, 2018)