I have to agree it would have been far better to include China or India in the game than the Aztecs. It just doesn't seem to fit the time. The Americas were only discovered at the end of the Middle Ages, and the great campaigns of conquest happened after the period had ended and the Renaissance had begun. The Americas had no real impact on Europe until after the Middle Ages.

I guess including the Aztecs will help Sega's sales in Mexico, but other than that, I can't really see any good reason for including them, other than eye candy.