Tlatelolco was a pre-Columbian altepetl, or city-state, in the Valley of Mexico. Its inhabitants, known as the Tlatelolca, were part of the Mexica, a Nahuatl-speaking people who arrived in what is now central Mexico in the 13th century. The Mexica settled on an island in Lake Texcoco and founded the altepetl of Mexico-Tenochtitlan on the southern portion of the island. In 1337, a group of dissident Mexica broke away from the Tenochca leadership in Tenochtitlan and founded Mexico-Tlatelolco on the northern portion of the island. Tenochtitlan was closely tied with Tlatelolco, and was largely dependent on its market, the most important site of commerce in the area.