Ken's Labyrinth - An FPS from Ken Silverman of the Build engine fame (Duke nukem 3D, Shadow Warrior, Blood etc.) ( youtu.be )
It’s not the best game, but considering he was a teen when he made this, still an impressive feat.
Later on he made the Build engine after seeing Doom and saying to himself: “I can make that”. This was notable enough that John Carmack, inhuman programmer of id software, gave him a call to ask how he achieved looking up and down in the build engine.
