Perilous Sanctuary [1]
The second layer.
Or rather, the true first layer.
The Severed World was called a sub-universe because its rules were nearly as complete as a true universe, but it wasn't nearly as big.
The entire realm was centered around a massive peak called Skyrend Mountain. With this in mind, Ghost King's Crevice was more of an entryway than a true layer of the ecosystem.
After all, it was completely different from the rest of the sub-universe.
Nevertheless, the second layer, the Perilous Sanctuary, was the stopping point for 99% of the practitioners who ventured into this world.
While the caves in the cliffside leading up to it were filled with Human-Faced Spider, the actual cliff wall was free of them.
According to Bertram, this was because the Human-Faced Spiders were prey for many of the beasts in the Perilous Sanctuary, and traversing the cliffside essentially made them sitting ducks in the eyes of their predators.