Pause Ahead Spoilers

Secret 1

Spoiler
On level 6, head up and wiggle yourself into the top left corner of the screen.

Secret 2

Spoiler
On level 13, make the Thwomp crush the floor twice. A door will appear where the face was.

Secret 3

Spoiler
On level 19, head all the way to the left and float below the bottom left spike.

All Secrets Collected

Spoiler
In the room before you face the architect, there should be an open passage on the left of the bottom floor.

Welcome to Nihil.