Aztec is a term used to refer to certain ethnic groups of central Mexico, particularly those groups who spoke the Nahuatl language and who achieved political and military dominance over large parts of Mesoamerica in the 14th, 15th and 16th centuries, a period referred to as the Late post-Classic period in Mesoamerican chronology.Often the term "Aztec" refers exclusively to the people of Tenochtitlan, situated on an island in Lake Texcoco, who called themselves Mexica Tenochca or Colhua-Mexica.Sometimes it also includes the inhabitants of Tenochtitlan's two principal allied city-states, the Acolhuas of Texcoco and the Tepanecs of Tlacopan, who together with the Mexica formed the Aztec Triple Alliance which has also become known as the "Aztec Empire". In other contexts it may refer to all the various city states and their peoples, who shared large parts of their ethnic history as well as many important cultural traits with the Mexica, Acolhua and Tepanecs, and who like them, also spoke the Nahuatl language. In this meaning it is possible to talk about an Aztec civilization including all the particular cultural patterns common for the Nahuatl speaking peoples of the late postclassic period in Mesoamerica.From the 13th century the Valley of Mexico was the nucleus of Aztec civilization: here the capital of the Aztec Triple Alliance, the city of Tenochtitlan, was built upon raised islets in Lake Texcoco. The Triple Alliance formed its tributary empire expanding its political hegemony far beyond the Valley of Mexico, conquering other city states throughout Mesoamerica.At its pinnacle Aztec culture had rich and complex mythological and religious traditions, as well as reaching remarkable architectural and artistic accomplishments. A particularly striking element of Aztec culture to many was the practice of human sacrifice.In 1521, in what is probably the most widely known episode in the Spanish colonization of the Americas, Hernán Cortés, along with a large number of Nahuatl speaking indigenous allies, conquered Tenochtitlan and defeated the Aztec Triple Alliance under the leadership of Hueyi Tlatoani Moctezuma II; In the series of events often referred to as "The Fall of the Aztec Empire". Subsequently the Spanish founded the new settlement of Mexico City on the site of the ruined Aztec capital.Aztec culture and history is primarily known:From archaeological evidence as it is found in excavations such as that of the renowned Templo Mayor in Mexico City and many others.From indigenous bark paper codices.From eyewitness accounts by Spanish conquistadors such as Hernán Cortés and Bernal Díaz del Castillo.And especially from 16th and 17th century descriptions of Aztec culture and history written by Spanish clergymen and literate Aztecs in the Spanish or Nahuatl language, such as the famous Florentine Codex compiled by the Franciscan monk Bernardino de Sahagún with the help of indigenous Aztec informants.