Fiercely committed to their sense of identity in spite of never having a nation of their own, they resisted invasion from their first appearance in world records in Roman times and always managed to negotiate commercial and legal independence, self-rule by the Fueros, administered under the ancient Oaks... lost at the dawn of the 20th century, viciously suppresed by Franco. They are still here.
The flawed second act makes the countess' peaceful retreat look like a bordello, the tippling knights disguised as nuns was well sung but unimaginatively staged, and Ory in nun's habit, wooing the countess in the dark, while unknowingly holding Isolier's hand, is turned into a tumbling threesome that doesn't come off, since it isn't at all clear which characters are aware of whom they are mis-grappling. But the first act was good enough to make up for the second.
I like alternate history when the point is to show that some of the things we take for granted weren't inevitable, but I think this story could have been told without the added conceit, by constructing a future culture that included all the elements he wanted to adopt from Aztec culture. Still, Harlan's extrapolation of Aztec society to technological imperium feels consistent and believable.