Our update begins with our scout discovering this: A stack of Hunnic units deep in Brazilian territory, and some city ruins. Gee, I wonder what happened here.
Hilariously enough, it appears that the only Hunnic units that survived the attack were some Hunters and a Scout - presumably because they attacked last, since Hunters have a city attack penalty. They all attack our Hunter, and don't even manage to scratch him.
A little while later, we discover Equine Domestication.
The Brazilians organize what might be a counterattack. I don't plan on sticking around to see if it is.
The AI really loves Atlatlists. Nearly all the units in this picture are Atlatlists. It makes sense, since they're Strength 4 when almost every other unit is Strength 3. The Atl-atl is an archery unit, but it's actually part of the "skirmisher" line of archery units, and gains bonuses against other archers. They can upgrade into bowmen, becoming mainline archery units, or they can upgrade into javelinists, who are the next tier of skirmishers.
That hunter is dead, by the way.
The people of Popumayapan have started building shelters made entirely out of snow. Seems a bit silly to me, shelters are there to keep out snow, plus they'll melt in the summer.
This might be because of that permafrost tile to the west.
And now, the moment we've all been waiting for... Megafauna Domestication!
Somebody else wanted us to research Caste System, so we're researching that now.
We can build bear riders now, right? Nope, we need to build training buildings first. Those are world wonders, so there can only be one of each.
Aberpanapopuma builds a Zebra Trainer, while MP4 builds a Bison Trainer. I didn't get a good screenshot of the latter, though.
Some centuries later, the Second Violent Migration is ready. It's mostly Neanderthal Warriors and Stone Macemen. Neanderthals have native Str 4 and a free Combat I promotion, while Stone Macemen have a city attack bonus. Hopefully bamboo armor and big bonuses versus cities will be enough to defeat the Persian defenses.
Now, the question is, should we attack the lightly-defended Susa, or the valuable Persepolis? Should we attempt to hold Persepolis if we capture it, or burn it to the ground because it'll be too difficult to administrate and defend?
Also, here's a comparison of Tribal society and Caste System society.
As you can see, Caste System is a usefuly early civic. The next civic, Proletariat, is unlocked by Democracy, which represents the Athenian model, and is quite a distance away.
Finally, have the tech tree.