Answer:
They built the city of Tenochtitlan on an island because it was the best choice if there was ever an invasion. Since it was on an island, the aztec army coudl protect all sides. It also had a system of canals and causeways that supplied the hundreds of thousands of people who lived there and helped protect the city. Whenever there was an invasion, the Aztecs would just attack anyone that tried to come down the canals towards the island.
Step-by-step explanation: