Optical illusions are something that deceives the eye by appearing to be other than it is. The eyes trick the brain into thinking something which isn't entirely possible but it's hard for your brain to figure it out.
Pillow Castle is a game that takes advantage of optical illusions. The aim of the game is to use optical illusions to progress through levels. The demo below is super cool and has many "woah" moments in it. Sure they're not your next gen graphics but boy does it look like fun.