In stage two, the jet mini-boss will drop a laser and a missile power-up upon dying. Pick up the missile power-up and don't die, or you will lose it. Keep playing until you face the pharaoh mini-boss in the same stage and beat him. Right after the fight, there will be a brain-looking thing on the right side of the screen. Shoot the dark door-looking thing coming out of the brain and it will open as an entrance to a secret level. You will also get "HAVE YOU SEEN IT?" achievement in the secret level.