City states can now conquer cities, as per Civ V

core/src/com/unciv/logic/battle/Battle.kt

@@ -307,7 +307,7 @@ object Battle {
         }
         city.hasJustBeenConquered = true
 
-        if (!attackerCiv.isMajorCiv()) {
+        if (attackerCiv.isBarbarian()) {
             city.destroyCity()
             return
         }