I'll try to explain the most i can't without it looking like i'm dissing the story.
First, i need to say that the premise it's a bit of a rip off from Tales of Demons and Gods, i need to say too, that the author changed it and gave his personal touch, so it's not much of a problem. Truth be told i like those changes, like the stories about the Spirit World.
Now, my major problem is the author has a lot of difficulties with the lines of his story, i mean, they're enclosed on a finite place, it's undestable that resources and more specifically land are really valuable, then we get that each member of the genius class get a radius of 75 miles or 150 miles of diameter. My country has provincies a lot smaller than that.
It's hard for me to undestand the limits of the knights on the history, the core of it. 13 years old children with punchs of 8 tons, ok, then the MC need to get out of the safe area, at 200 meters of the wall, the find a diamond storm, but they're super away to get back to the wall and be safe. Man... I'm not exactly atletic and can do that 200 meters in 15? 20 seconds? If the knight are so strong i think they're quick at the same proportion, being atleast between 10-100 faster than a normal person, but they prefer look around for a cave than go back 200 meters with this superspeed. Author you need to look at the number better, maybe 20 o 30 km or even more will be a lot more understable on this plot.
Characters are lot better truth be told, they're are a few of the good points of the story, and then he ****ed it when he lost everything he had of the novel and thought why can i not change this in first person? Then the MC changed from an ancient protector to a warmonger arrogant child.
The last thing and what i love the most of this type of novels, reincarnation type i mean, are the details of the world, things that the people doesnt know, because it's something extremely ancient or special that they think it's simple or simply didnt undestand. At first this was a strong point here and then it simply stopped, where was everything? I liked that a lot.
I'll not say much about the fault of editing everything in the last 30-40 chapters but, yeah, that's a big "-" on my eyes too.
With some work this can be remade on a good novel.
Good luck