6 barbarian horsemen spawned as part of a random event outside an ai civ's borders... years later, they arrive at one of my cities... i have axemen and spearmen fortified... the next turn, my city is owned by barbarians, my worker is gone, the barbarians have moved on untouched by any combat, my units have been displaced untouched by combat... there isn't some sort of barbarian rampage intimidation super secret city stomping power is there?