Hi Chema,
This game is really interesting, I was hooked instantly from the title and cover art! I feel like I have to say, I was a one time GM who tried to rule a DnD campaign, but got discouraged by the overwhelming amount of rules, so the simplicity of your game and the setting got me really excited to try to put a game together again.
One bit of criticism though: After reading the rule book I still had a lot of questions, a lot of sections give me an idea about how some rules work, but they feel incomplete, like I missed something. My biggest question was the same as Quartzio, I wasn't sure if it was just me so I had to check.
The rules seem quite unusual, specially for people who never got into any game besides DnD, so I think the book would benefit greatly from a few examples. Maybe even one of those "game demos" some books have.
Stuff like the monster's "AC" being first tied to 10+hit die, then dropping as it takes damage is quite straightforward when you get it, but I had to read that section probably 3 times before I understood what it was saying. I think a walk-through section narrating monster creation and combat would solve all of that.
Also, one last question, what would be your suggestion to calculate the damage from the Kotengu dropping the player?