Final answer:
The Aztec civilization, or Mexica people, inhabited Tenochtitlán when the Spanish arrived. They had a highly developed urban environment, but the introduction of diseases by the Spanish and alliances with other indigenous groups contributed to their downfall.
Step-by-step explanation:
The people living in Tenochtitlán when the Spanish arrived were primarily the Aztec civilization or Mexica people, as they were also known. When Hernán Cortés and his Spanish expedition arrived in central Mexico in 1519, they encountered an advanced society with a large population, sophisticated urban planning, and bustling markets. Tenochtitlán, the capital of the Aztec Empire, was a thriving city and one of the largest in the world at the time with a population exceeding two hundred thousand people. The Aztecs had a unique system of artificial agricultural islands called chinampas, grand temples such as the Templo Mayor, and impressive causeways connecting the city to the mainland. However, the Spanish arrival brought diseases like smallpox, which played a significant role in the Aztecs' downfall, as well as military conquest aided by alliances with other indigenous groups like the Tlaxcalans who resented Aztec rule.