I'm at the Hallways of Insanity and I can't find the last switch with the sword icon to raise the steel fence where the checkpoint is. Where is it?
It's inside a hidden room, located around dead-center in the middle of the floors. You can identify it because you've probably used the skill to slide down underneath it, as there is a long 1-space-tall tunnel under it. There are 2 ways to enter it, either from above or by pushing a box to the right and sliding inside.