Countless cities and small city-states exist in central Mexico; populations increase substantially. The city-states engage in constant warfare while trying to expand their political and economic base. Most are unable to establish themselves in larger domains or empires. Long-distance trade grows, and traveling merchants have an important place in the social hierarchy, as do professional soldiers. Stylistic features and symbols spread throughout large areas. About 1200, numerous Nahuatl-speaking groups from northern Mexico migrate to central Mexico, where the Aztec culture will coalesce. One of the northern groups, the Mexica, rises to prominence in the fifteenth century as rulers of the Aztec empire.