Doom is one of the greatest games ever made period, created in 1993, Doom revolutionized 3d graphics thanks to a whitepaper John Carmack once read from 1980. The game was written in C and used wad file format for maps. It layed the foundation for all future games of the first-person shooter genre (just like how Vi did for all text editors to come), and to this day the gameplay IS an absolute blast (and also very easy, circlestrafing is so easy my friend was able to do it).

Game Engine

The game is written in C and thus in certain groups is viewed as the only real suckless game engine. Well until drummyfish's Anarch. The game's engine is licensed under the GPL version 2, this is good albeit if it was something permissive then copyleft (ideally public domain due to the mere technological impact of the game) would be better, despite that, this means any game made with it is FOSS. However, Doom is not FOSS entirely albeit it's probably 97% free software due to the data being proprietary and the MIDI music probably also has some copyright in it

Fun fact: I was originally gonna make a whole GAME in the Doom engine, via Zdoom source port, I even got my friend to even use it. Until we realized we couldn't just make up our ideal game and well I'm not really sure at the moment. Either Castle or Godot as both can do 2D and don't seem to need to be compiled. When I stopped devolopment and actually played my pk3 it was well, fucking dogshit as GZDoom stated I was using the wrong version, I fixed this but now if you try and play the pk3 it will state 4 errors so please only extract and then play the wad file in it, as that one actually works.

Also if you have any wonders about what game engine to make your game in. I implore you to just use Doom. Anything you could ever need is within Doom's capabilities.

Wadding

Doom is one of the few games I could never live without modding. Wadding is what proves Doom to be the best engine, every Doom wadder is a seriously talented individual with impecable creativity, take myhouse.wad, Aliens TC, etc... Doom is a masterpiece in every single part. The gameplay stands to this day, the engine can still do thousands of things showing you don't need Unreal or Godot bloat to make something awesome, the music is amazing, the arsenal is phenomenal, the community is creative, the graphics are great, new features are added all the time to it (ZScript, UDMF, source ports), it's portable, it runs on anything, and the source code is free and thus, nothing stops me from ever being able to play it, it is what games should strive to be, it is, eternal.