What was the Aztec empire's social cl system like? and more...?

In many respects the Aztec (Mexico) empire was devloping into a relatively modern state society by the time Cortes Arrived on the shores of Vera Cruz. There was a sophisticated social hierarchy but it was beginning to be challenged by a particular segment of that hierarchy. What was the social cl system like and how was that cl the greatest challenge and potential for change of the traditional mexica hierarchy