So it actually is possible to make Doom on NES.
It's incredibly impressive!
I've seen other attempts at 3d fps games on a variety of 8-bit systems, but none of them even attempted elevated floors.
The graphics could probably be improved by making it less blocky, but it would take a huge drop in framerate, because of the processing power needed to generate new tiles on the fly.
Anyways, it's an amazing tech demo.