I wish I had that going on. Like some kind of virulent plague. So far the AI in my games has been fairly capable of keeping them at bay and even recapturing cities from them.
Also using the Barbarian Immersion Enchancements which gives them great generals you can capture for Gold and Culture, scales their name with eras, and gives them perks in later eras.
To top it off I use a mod of my own design that makes cities much less dangerous and all but removes the damage melee units take on attack.
The cocktail of three mods keeps things interesting.
In my experience the Barbarians get more and more dangerous the longer they are ignored and the more cities they take.
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I'm Spain with Babylon and the Mongols for neighbors with China in the north - only civs on the Communitas continent.
Its cluttered down south, not helped by my seizure of natural wonders, and for a time I just couldn't spare troops for Barbarians due to a war with my neighbors.
The brigands took a city-state in the north early and one in the south recently, and China actually fell to barbarians, though I liberated them eventually.
(And at this rate, might have to liberate them again.)
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So the north was a massive uninhabited barbarian land that took me
forever to clear since they had been spawning for a long time, and there are still barbarians in the north east that will require a real army.
Meanwhile that city state in the south falling has led to Babylon losing the capital, which I've liberated for them.
It's forcing me to devote my forces to the area - too close to my own cities.
This isn't made easier by my army consisting of Conquistadors while the Brigands are primarily using Musketmen - really in need of Cavalry soon, but tactics can win the day.
Outside my continent, pretty much everywhere not immediately occupied by another civ is overrun with barbarians.
It's a problem I've never had in Civ before - too many enemies to fight!
And they keep respawning!
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