“To win the game you must get 100% on level 15” -
https://twitter.com/romero/status/1035609232501030912
https://mashable.com/article/doom-2-secret-24-years-found
ref - https://www.tweaktown.com/news/63026/doom-ii-secret-found-24-years-hiding-plain-sight/index.html
ref https://www.youtube.com/watch?v=irNoHfnLXRM
origin - https://www.pipiscrew.com/2018/09/doom-ii-secret-found-after-24-years-hiding-in-plain-sight/ doom-ii-secret-found-after-24-years-hiding-in-plain-sight