Simplify the following text for a Grade 6 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 .
Simplified: The modern day city of Mexico was built on the ruins of Tenochtitlan .