Simplify the following text for a Grade 8 native English-speaking person.
Original: Tenochtitlan ( sometimes also known as Mexico Tenochtitlan or Tenochtitlan Mexico ) was a Nahua altepetl ( city-state ) located on an island in Lake Texcoco , in the Valley of Mexico . Founded in 1325 , it became the seat of the growing Aztec Empire in the 15th century , until captured by the Spanish in 1521 .
Simplified: Tenochtitlan was the massive capital city of the Aztec empire , built around 1325 .