I'm playing as America, and this is in around the classical/medieval era, and Aztec declared war against me for no reason at all. I had the worst army in the game but took over 4/5 of his cities before he surrendered.
Then there were notifications telling me that there are enemies spotted around one of my cities, and it turned out to be 2 barbarian longswords men, and then there were 2 more spearmen that spawned at the same place, then there were 2 horsemen spawned in the city to the north of Aztec's capital.
Is this a glitch? I have 23 unhappiness, and it fluctuates between that and 21 very often, and my income fluctuates between -2 to 8.
Never had this happen before. And it seems that Barbarians spawn in pairs, and they don't spawn more until the pair is killed