Even though the world was a place filled with monsters and demons, it didn't mean there weren't any highly intelligent people around. In the outer area of the place, everything was filled with broken human buildings and a lot of brainless monsters who only knew how to attack their enemies and nothing else.
As one started going deeper into the area of the monsters, they would see small huts on either side of the roads. It was a sign that dwelling places have arrived. By this time, the monsters would be reduced to the point that one would only see small animals wandering around the place.
The dwelling huts were filled with weak demons who didn't even dare to go out of the house for the fear of spotting the monsters. At the same time, they didn't have enough money to head over to the city area. So they couldn't help but live at the borders between the monster and demons' area.