I've been playing some more, but started a new game. I stuck with Noble and this time I'm owning my neighbours playing as the Arabs, but I must lift my hat off for the AI. I had previously declared war on the Aztecs to lay claim on an area they managed to settle only two turns ahead of me (I'd done the dirty work and razed a barb city just to get to that site). I actually founded the city next to the ruins (because the AI wasn't quite as clever when picking his city site, going for a spot two tiles from the ocean which is a real no-no).
The population of the city I founded (on the coast) was actually 3% Aztec and 3% Barbarian, so the game remembered what cities had been around the same tiles earlier, which was really ultra cool (I had to go to the loo and...). What was even more stunningly ultra cool was Montezuma's course of action a good while later. I'd reinforced the border cities (the other one was a bit further up north and safely out of reach, or so I thought) and gone about my business without fearing the backward Aztecs particularly. I even neglected my army a bit and went for wonders and other buildings.
That's when Montezuma declared war and he actually went for his old baby instead of the closer cities. What actually really impressed an old kook like me locked in a Civ I-III frame of mind, though, was that he made a massive sea assault with three galleys and several military units plus a settler! All of a sudden I had three horse archers and an axeman standing safely in the woods outside a city I thought was well out of danger (and thusly only guarded by a single axeman). He didn't stand a chance and now I'm gonna have to try and retake the city, which actually might prove tricky because of the lack of quality units I have in the area. Nifty!