This screenshot happens to be from my FF+ game, but you still see this often in FF too where the AI will inexplicably pick a city (not always Acheron's) and just load up on defenders. In this case there are 17, but I have seen more. Yet, often you see the AI barbs capture a city and immediately move out the units allowing it to be re-captured. Why not leave a few defenders, or even just raze the city instead of giving it up. Honestly, my vote is for the barbs to raze EVERY city they take. They can always spawn new ones any time (and they do prolifically), so why not let them level any they take? Finally, by letting the AI raze everything and move on, I think you would have a much better raging barb game. You shouldn't see them with 17 defenders in a city either - unless maybe it is Acheron's. Maybe it is more balance than bug, but I think the bug part comes in when you have the barbs just vacating many cities they capture and leaving them wide open.