This is just amazing. And what is more suprising is that Doom 1 and 2 are still thriving after >20 years. If you download zandronum you will learn that there are still people playing it online. of course those are some mods, but still very much resembling the original atmosphere and graphics of doom 1 and 2.
Edit: of course this probably would be different if the doom source code wasn't released at some point of time...