The layered floors that lie within Scolopendra's reach; only the sixteenth sits outside them.
The planetwide evacuation announcement drew a hard line through the dungeon's lower floors. Because the sixteenth floor, Agony Mirrors, sits outside Scolopendra's layers, it is the only place guests of the Scolopendra Club are safe from any impending attacks.
Everything else — including the seventeenth floor, The Backstage Death Maze — remains inside them, which is why crossing that maze is the price of escape. Moments later, Scolopendra awakened.
The structural fact that makes exactly one floor safe and turns every other into her territory.