User blog comment:Armada 371/Maruyama Interview translation from reddit./@comment-33177490-20170527204415/@comment-5480911-20170528102811

Their are other factors at play as well such as natural barriers, the Slane has the elven nation to the south and beyond that is a vast Desert that is difficult to travel trough it and the north has the mountain range that borders Re-Estize and the Alliance.

Also the level of mosnters seems to gradually increase, so it is not like their is a sudden gap in power. to give an example look at the intermission in vol 10 regarding ST and Elven nations war. The reasons why it has not ended after so long is cause of the elven nations guriella tactics so they are using their terrrain for an adavtange alos the monsters their are much stronger on average than the ones in the forest of tob. the scriptures can still handle them but can overwhelm instantly.

Think of it like your standard mmorpg. you start of in areas with low level enemies but as you further out you go, the more difficult the enemies become.

Like say one area is max lvl 30 the middle zone is lvl 35 and the other lvl 40. the middle one can balance things out were as normally the higher levels would be the lowest levels. i think that is how it works.