Whats really weird happened with my last game as Mali, I saw some barbarian cities with the Buddist Religion. I thought it was odd since I was playing Hemispheres and I never saw Buddism founded in our half of the world. I also thought it was odd that they had plenty of space in their own area of the map almost as if they had founded their own civilization.
In vanillia civ4, back before when Stonehenge went obsolet with the Calander, I saw the barbarians build it one or twice since noone else was still able too.
I have experienced both things. My latest fun way to play civ is me vs the barbarians. The way I do it is start up a game with a few rule changes:
1) I'm the only civilzation.
2) Raging Barbarians.
3) Pangea, size small or greater.
4) Only winning condition is Domination.
And I have two self imposed rules:
1) No building the great wall (that's cheating!)
2) No building spies.
It's tougher than it looks. I played through a noble game last night, and I have to admit, starting off is pretty tough when the ONLY copper in the entire continent was in the center, and I started off on the far eastern section of the continent. ... and the Barbarians built a city right on top of that copper!
I nearly killed my economy building up an army to 1) Survive the trek halfway across the world to claim that copper while 2) protecting my poor worker who had to build a road there and 3) make sure the army that made it there could not only take the city but 4) defend the city as well as 5) defend my own lands. For a while I was sitting at 0% science and still losing money!
Regardless, once I got there, I saw some roads leading out of the city. After taking the city, I sent a solo archer to investigate. That archer traveled to the far southwestern corner of the continent, and what did he find? A size 6 barbarian city with an impressive culture radius! A bit of wandering around and I discovered another road headed north out of that city. Being adventurous I followed it to yet another city in far northwestern part of the continent. But sometime between then and reaching that city "Confucism has been founded in a distant land." "But I am the only civilzation," I thought to myself. After scratching my head for a moment, I realized that the barbarians just founded Confucism! This city to the far northwest was where the barbarians founded Confucism! Not only that, but once I got there, the city had all 20 squares surrounding the city with cottages, farms or mines. And the farms were on the rice to get them the health bonus!
I was very impressed with how much the barbarians acted just like a real civilization.
I did very well against raging barbarians on Noble, perhaps too well, and now I'm thinking of kicking it up to Emperor+ to see how well I can manage against the barbarians with the penalties that occur from the higher difficulty levels. But I will say this much, barbarians are a tough crowd because they are basically a regular civilization that you cannot ask (read: bribe) for peace.