I have an idea. Play on higher difficulty, have The Celts as your neighbor, settle your city next to them, ask them not to settle near you, denounce them. Just for fun. Then, watch Boudicca declares war on you few turns after that, sending dozens of units, slipping settlers to "that" empty space right next to your cities. Not kidding, happen to me. Even her missionaries were involved in war. She send lots of missionaries\prophets to convert my cities so I wouldn't get all that extra happiness from my religion (+1 hap from shrines, +2 hap from temples) Oh yeah, and then The Huns decided to randomly declare war on me, even if Attila was on the other side of the map.