<human>:  When did the Aztecs start to build the city of Tenochtitlan?
<bot>: The Aztecs started to build the city of Tenochtitlan in 1325. They built it on an island in Lake Texcoco, which was connected to the mainland by a causeway. They used a mixture of mud, clay, and straw to build the city. The city was surrounded by a wall, which had three gates. The city was also surrounded by a moat, which was filled with water. The Aztecs lived in the city for many years, but they were eventually conquered by the Spanish.