Uta was a large kingdom that covered millions of acres of land on all sides. The majority of the kingdom was occupied by forestry that formed a wide u-shaped covering along the outside of the kingdom. The forest outside the kingdom was out of bounds for anyone that didn't have at least D-class skills because it was filled with demons that made it into their home.
The demons in Rivalle were very free-spirited. Even after Floid came to Rivalle and announced his presence as the new demon lord, there were still a lot of demons that decided that they wanted to continue living in the forests that they settled in. The number of demons that remained was very few, but they were still enough of a threat for any small town or village. Most of the demons that remained in Uta were goblins, but there were also giants and even some dangerous lizardmen or minotaurs living in the forest.