So, I decided to try my hand at creating a Doom level...
I recommend you play it via a Doom source port (e.g. GZDoom). I haven't tested it via DOSBox - but DOSBox is generally a bad way to play Doom levels in 2018 anyway.
Just download the level and drag the file onto GZDoom (or your source port of choice).
Thanks to shami_sansi for feedback.