I Started a new game in a mounted area. By the second raid the raiders had the great idea to pass throw a cave where some nice hungry bugs where living. That day the bugs had a nice diner. So later I made a wall on that side of the map with a big hole in my wall. In that hole the bugs are living happy and well fed(until raids are to big).
Edit_____________16/1/17
Raider have killed them all :(
That's some nice natural defense you got right there