I've tried the following setting: huge map, only one AI as Venice, raging barbarians, one-city challenge, no city-state, level 5. I expected the barbarians will capture the capital of Venice because of the number of encampments and the high spawning speed. Also during the game, Venice has built Great Library and Parthenon. So I think he was not on the track of military sciences. But even with this, I didn't see the barbarians capturing his capture. So is it possible to see barbarians conquering an AI?