When the Spanish under Cortes first encountered the indigenous people in Mexico, which feature of the Aztec Empire made the greatest impression upon them?

Cortes and the Spanish soldiers he brought with him were greatly impressed with the Aztec Capital: Tenochtitlan.

Explanation:

Tenochtitlan was the Aztec capital: a large city build in the middle of a lake: the lake of Texcoco.

Tenochtitlan was connected to the mainland by causeways and bridges, and had aqueducts made of terracota.

The Spanish marvelled at the city, and this motivated Cortes to meet the Aztec Emperor: Moctezuma.