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I think that the goal of the project is not "Doom running on a 40-Cortex M4" (actually an 80 M33...), which is pretty easy I guess, but "Doom running with only 108 kB of RAM", while keeping all the features (which is pretty hard, I guess). I recall that I had to bend over backwards to get it running on my 386 with only 4MB RAM.

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The game is cheating a little bit, since it loads a lot of read-only data from external SPI flash memory, and all the code is in the internal 1MB flash. On your 386, everything including the OS had to fit on that 4MB RAM.

It also doesn't have quite all the features. No music, and no screen wipe effect (I worked on a memory constrained Doom port myself, and that silly little effect is incredibly memory intensive since you need two full copies of the frame buffer)

Overlays were a thing in DOS days, as well. Not for Doom specifically, but I've seen quite a few 16-bit games using them.

They're too slow for action games like Doom.

Not everything, only what’s relevant to the individual levels.

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