A technical trick
Computers in 1993 could not handle true 3D. John Carmack wrote an engine that drew maps as a 2D floor plan with heights. The result looked three-dimensional and ran smoothly even on an ordinary 486 PC.
Shareware and mods
The first episode was free and the rest had to be bought. The game spread on floppy disks and bulletin board systems. Level files (WADs) could be edited, so a huge community of creators formed. Mods such as Counter-Strike later grew out of this culture.
“Can it run Doom?”
Doom's source code was released in 1997. Since then people have run it on calculators, printers, ATMs and even a pregnancy test with a display. The series continues with Doom (2016), Doom Eternal (2020) and Doom: The Dark Ages (2025).