http://i.imgur.com/frCkc.jpg
http://i.imgur.com/3GW1N.jpg
Basically I remembered that someone, in some level, hit some keyboards and they made a sound. Hit enough and a Demon made a noise and then you opened some door and there was the Doom room. Recently I made it to the Gearhead level saw a keyboard, hit it and it made a noise. I hit enough and a Demon made a noise. I beat the level and on my way out the level I tried a door...
http://img.gawkerassets.com/img/17f1hlyaxcpstjpg/original.jpg
Nice.