The Grumpy Celt said:
It was, and remains, a pity the Mazteca setting was handled so poorly.
True.
The Grumpy Celt said:
Fagan’s work is unpardonably elitist.

This statement honestly confuses me. I've read
The Little Ice Age and
The Long Summer and neither come across as elitist. He combines climatology, archaeology, and history to better explain how climate and its ever continuous alterations has encouraged or discouraged / stressed human civilization at various points in history. His style of writing is readily and easily readable, using vocabulary any high school graduate could understand. Both books are as quick and enjoyable a read as
The Elegant Universe by Brian Greene,
Seiges of the Middle Ages by Philip Warner, and
Gifts Differing by Isabel Myers. The only potentially elitist author of Climate / History comparison books I recall offhand is Felipe Fernandez-Armesto, and that was do more to word choice and complexity of sentence. (It was not necessarily meant for the lay reader, or at least the book I read didn't seem to be.) Anyway, sorry for taking the thread off topic for a moment with this. It just confuses me. Are you sure we are referring to the same author?
The Grumpy Celt said:
Why would they be good for the game he is talking about?
The former (frost and fur) might be of use in that it details some pre-metal type weaponry. The latter is better in my opinion, as it does not focus upon ice age technology. Granted, the late stone age tech was little better than that, but there is a difference between such tech in an arctic environment and a tropic (or subtropic) environment.
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As to how I would run a mesoamerican setting, I wouldn't. Not exactly anyway. I would create a highland area in the northern part of the tropic region. It would have several shallow lakes and placid rivers / streams - perhaps a slightly steamy atmosphere in early morning / late evening during the wet season (not a true rainy season). It would be just far enough north so that grasslands are starting to replace jungle. The region would be poor in harder metals, but softer metals, like gold and copper, would be present. The lack of tin would prevent bronze. Thus the softer metals would be mostly used for decoration. The predominant stone used for building would have a reddish hue, likely being a sandstone of some type.
The people would have a red-brown complextion. They would be advanced enough that elsewhere - with a supply of tin - one might expect an early bronze age civilization. I would have a creature similar to a cross between camal and llama for a draft animal. Not strong enough or large enough to carry a person, but more than sufficient to pull small wheeled carts and carry small loads - even children. Wheeled carts would not be used during the wet season or when traveling to the low lands, as mud and steep / rugged paths would make them less useful. The draft animals mentioned would be used without them during that part of the year or when travelling to those lower land regions.
The highland plateau would be high enough to allow for warm temperate weather rather than true (sub)tropic weather - although summer would be obviously tropical in intensity. A weak but certain wet / rainy season around late summer / early autumn, would suggest at the monsoon conditions of the lowlands nearer the ocean - a region of mild jungle visited more for hunts and trading with sea shore cultures than for any other reason. The planting would require irrigation due to the drier conditions of the late winter / early spring, and this would lead to a social heirarchy developing (much as in the fertile crescent and along the nile).
A firmly defined class system would exist: farmer, warrior, and priest. The king would also be the high priest, further emphasizing the role of the gods in daily life and social organization. During the non-growing seasons the farmers would double as labor for building temples, etc. Artisans would exist, but they would be a minor class usually treated as part of the farmer class, except where weapons / armor and religiously important items were concerned. Those artisan families associated with such would be considered subsets of the warrior or priest class (as appropriate).
Merchants would be specific to what they traded. A merchant peddling shoes would not also be peddling arrows. Those are merchants from two different classes, after all. The exception would be merchants involved in copper, gold, and gems. Those would be mined / panned and paid as tribute (in lieu of labor either in fields or in building projects) to the local priest/kings, and merchants from the priesthood would be the only means of accessing such - after the high priest/king and those nearest him have already taken his selection and due from the offering / tribute.
City-states would exist, each holding lands within a few days travel of their main city (or more properly, large town - at least for most of the the city-states). They would vie for dominance and land, as such was important for food production. During peaceful times the warriors would be hunters, taking forays lasting days or even weeks into the jungles of the low lands, perhaps having the younger generations undergoing special hunts as part of their coming of age ritual.
Literacy and recordkeeping would exist, but such would be the province of priests. A simplified written version of the sacred language would have eventually become known, and it would be commonly used by merchants and some artisans to keep records, but such would be discouraged in some of the citystates, which might see such usage by non-priests as defiling the sanctity of the literate language. Others might encourage this, so that laws and sacred edicts could be posted and read by others - although this would be an extreme view held by few (if any) of the city-states. Rumors of such could be used as reasons for initiating combat, however, if the lands held by that citystate were desired.
Because of the yearly rains (albeit not rain forest / monsoon extreme) the soil is less than truly fertile. The rivers, slow moving as they are, take sediment away from the highland plains, ever weakening the soil. True, some sediment comes from nearby peaks where the rivers have their source, but it is barely enough to make up for what is taken. Irrigation is labor intensive, and even with it crops are not always as abundant as wished. Too little rain early on may cause rivers to run low, and too much later may destroy crops in the field before they can be harvested. Deities are seen as less than orderly - although not as capricious as those in Sumeria were viewed to be.
Bloodletting is a typical part of the religious ceremonies, fertilizing the land with the life-filled blood of the high priest (and others, upon occasion). During the most extreme years of drought / excessive rain more severe bloodletting might occur - to the point of human sacrifice in some city states. Such may be reluctantly accepted and even seen as necessary by allies and/or reviled and seen as defiling by enemies, furthering local inter-community tensions. Prisoners of war, local great warriors of renown, and/or local high ranking priests would be selected for such an 'honor'. This would, however, only be a real possibility (in most of the city states) during the worst years. Most would have just a little occasional bloodletting at the various festivals (during good years).
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Okay, on the fly, that is how I would create the base setting. Further expansion would be necessary, of course. Details of the religions/mythos and when their festivals occur, when the last good / bad years were and whether or not they tend to come in cycles, the number, strength, and degree of interaction between the various city states (and also the shore and jungle peoples of the low lands), and any current alliances / feuds of note. Also, any interesting creatures in the peaks, highland, jungles, sea, and perhaps even underworld might be of interest - semi-mythic creatures that the PCs might meet. I would also have to set about determining which classes are allowed, the necessary adjustments for the setting, as well as potential adjustments to skills, magic, races, creatures, etc. I would state that the highlands are a karst formation just forming, so that some (but not too many as of yet) cave systems might exist, allowing for some underground adventures / creatures / races, perhaps newly moved into the area (within the fast few generations). Then I would be read for the adventure.
Now, you stated you wanted an invasion type situation involving a portal? I would have the portal form on your side (not the psuedo-mesoamerican side) of the interplanar boundaries. Recalling that in d20 the plane of Shadow is used to connect alternate worlds, I would state that the portal opened up in the underground / cave region. Perhaps a major ritual on their part went wrong, and the surge combined with an attempt to create a teleportation circle type gate on your side, creating instead a stable portal through the deeper shadow between two worlds. The underworld sapients fled upon seeing the odd vortex opening, collapsing the entrances to their tunnels in an attempt to prevent following. Explorers from your side found an odd chamber, perhaps with a skylight from an overhead opening. Eventually working their way up and exploring the surrounding area, they eventually found one of the city states.
Now their presense has upset a delicate balance of power between various more powerful city states and their allies / tribute citystates. Worse yet, the current year is drought prone, and some are stating it is interaction with the odd foreignors that is the reason. Your adventurers discover later that one of their 'bosses' is in fact in secret communication with the primary enemy of the city state they currently interact wtih. It seems he is trying to further unbalance the situation, hoping for war that will crush one side and weaken the victor enough for him to easily take over after. They can assist, but even their new friends / allies are suspitious of them somewhat. Warning them of the danger from their 'boss' might further convince them that having any interaction at all with the foreignors is bad. At the same time a struggle is forming within their friendly/allied city state. A high ranking priest, distantly related to the current high priest, is plotting to overthrow /assassinate the current high priest, and he is backed by yet another enemy city state.
This would be how I would set up such an adventure, when limited to the pattern you prescribed. Personally, I would not use a portal. I would have the adventurers either starting in the city states or being a Lewis & Clark type expedition from another land further away. Imagine if the americas had first been landed upon in the north (around Canada / US border), and the rest of the americas were effectively unknown until the time of L&C - then an expedition was sent south. Of course, to have the tech about right, I would have the americas actually settled (successfully) at the time of the vikings. others would follow in their wake, but the little ice age would cut off voyages for a few centuries, during which the colonies - at a medieval level - would grow and develop.
A few centuries later - after significant independent development but before communications were restored with europe - the L&C similar expedition would occur to further explore the southern lands. This would mostly be due to ever present clues of trade occuring between their southern most neighbors and some more distant land to the south that seems to be more developed than local tribes - and knowledgeable of precious metals such as gold (and to a lesser extent copper) and precious gems (recall the emeralds of the incas - I might as well put such mineral wealth in those mentioned nearby peaks, and pearls / abalone in the sea).
Anyway, that is how I would set up a psueo-mesoamerican adventure. Nothing that screams "aztec" or "inca" or "mayan". Just a setting that suggests a similar overall feel while being different enough to still be distinct. And the bit about the more northern setting of the americas? That was only for a RL example. It could simply be that a more northern land with a medieval setup is only just starting to move southward, prevented prior by some means. Perhaps a mountain range blocked it off, the single pass held for centuries by a dragon that has only just recently been slain, finally opening the way to new lands. Numerous reasons might exist.
As can be seen, I prefer a more realistic / lower magic world. Not necessarily low magic. I don't mind magic being common, just limited from reaching higher levels. Perhaps wild magic surges discourage it, or the deities do not allow it, or the knowledge is not yet gained (like steam tech and electricity to the ancient romans; or more apt: calculus to the ancient greeks).