First time I've seen that. Kongo beat me to a city spot. I'm playing as Gilgamesh, so I've been fairly aggressively killing barb camps for the bonus, but I left one that was a lot closer to this city than to my city that was 4 tiles away. It was also close to another spot I wanted to settle, and I thought they might keep Kongo settlers away, or even capture one for me. A barbarian scout found me and soon barbs were pouring out and coming after both of us but mostly him; the camp was close to some iron, so they had swords. Next thing I know, the Kongo city is gone! (I think it had walls, too, but I'm not sure) I mopped up the barbarians with a knight and a crossbow, and I noticed they had a battering ram and catapults. I eventually settled a city where that one was destroyed, and another where the encampment with the iron was. The barbarians saved me a lot of trouble; I thought I was going to have to goto war and take that city. There will be a war soon (Mvemba has been denouncing me over and over) but I can take a much better city that has the Pyramids instead of this outpost that was in a good spot but hadn't developed yet.