The Lord of the Rings Yahoo group has army lists for playing a Aztec v Conquistators variant.
New World DBA is a set of amendments and lists for playing DBA games in the New World: Tupis, Aztecs, Incas and Conquistadors
In the Jungle is an interesting set of rules for conquistador vs aztec games. It’s played on a gridded board and is supposed to simulate a group of “lost” conquistadors trying to find their way to a temple without falling victim to sacrifice.
Obsidian Blade: Twilight of Old Mexico is a set of free wargames rules for the Spanish conquest of Mexico.
This is an introductory set of wargaming rules. It is designed for the novice player with little or no wargaming experience. Surprisingly, the simplicity of the rules has added some nuances of play that often escape more experienced players. The first is that this is not a simultaneous move and fight game. Consequently, deciding whether to move first or second can be a major decision, particularly for the Aztec player. The ideal situation for that player is to go second in a turn and manage to surround some of the enemy, then go first in the next turn and hustle the captured figures away for sacrifice at a later time. It is to the advantage of the Spanish player to ensure that the Aztec player never gets to move two consecutive phases. Secondly, it is important to realize that hand-to-hand combat is not simultaneous and that it is generally better to attack than defend.
There are those who will decry the simplicity of these rules. Yes, victory depends on a certain amount of “gamesmanship” rather than sound strategy. But then what do you expect for free, on the internet, and on less than two printed pages?