2nd comment because I realized the "climbers" are probably the creatures coming up from the crevasse! Because... they're climbing... up... to EARTH!
booooom.
Just how far down in the "tower" are those monsters coming from? Did they start from the bottom and wipe out 2-8 or are there worse things even lower? What is on the higher floors? Interesting twist.
So earth is the 9th level of this weird tower system. So now our MC is the guardian against these so called climbers. So how would a "climber" advance to the next world?