Did the movie state the time period at the beginning? I seem to remember it saying something like 600 years ago, or something that made me go 
when they showed the Spanish ships at the end. And I remember saying to myself as the camera showed amazement on their faces when they were standing on the beach: Please, please, please don’t have the Spaniards arrive!!!
I don’t recall there being any statement as to time period.
The movie opened with a quote from Will Durant: “A great civilization is not conquered from without until it has destroyed itself from within” - something that was clearly not at all true in the case of the Aztecs or Incas, which were going strong at the time of contact but essentially were conquered by European diseases.
Given Gibson’s religious attitudes, it seems clear that the Spanish were brought in to suggest that idea that Christianity would save the benighted natives from their repellent religion. Of course, when you knock back the population by 90% it does tend to cut down on environmental degradation as well.
A brief interview with Hansen published in Archeology magazine about Gibson’s movie can be found here.
And a longer feature-length article about Apocalypto from the same issue of Archeology is at this link.
I may be mixing it up with the trailer I saw for “Pathfinder”, which was shown just before “Apocolytpo”. I’m pretty sure a definite time period was given for the former.
Was it not the resentment of some of the subject peoples that enabled Cortes to easily gather allies and topple the Emperor? Of course, if that is true it would be a classic case of not picking your rescuers carefully enough.
Yes. The Tlaxcalans and others (who were not actually part of the Aztec Empire but were surrounded by it) allied themselves with Cortes on his initial march to Tenochtitlan. They probably had the idea that Cortes was helping them to overthrow the Aztecs, whom they would then supplant as rulers of the Empire. The joke was on them.
Cortes would almost certainly not have succeeded without his native allies. After the death of Moctezuma, the Spanish had to flee Tenochtitlan with great losses on the Noche Triste. Once outside the city, they regrouped and rallied their allies. Cortes laid siege to Tenochtitlan and eventually took it, aided also by a smallpox epidemic that broke out among the Aztecs. The source of the disease was one of the African slaves among Cortes’ men.