The speaker argues that Montezuma's mistake was not inviting Cortes into the city, but remaining in the capital and an adjacent palace, which made him vulnerable.

“So the emperor's mistake wasn't inviting Cortes into this.”

“into the city he was making himself vulnerable by staying there also in an adjacent palace if he had gone to some further location made some excuse a pilgrimage or an expedition of some kind and had ruled through subordinates Cortes would have been baffled and he couldn't go chasing after the emperor that that would be obviously too provocative but without the emperor there Cortes would have had his options extremely limited and I don't know what he would have done in that circumstance.”