First off, this is a fun reference and honestly seems quite good at capturing the doom feelings in 2D. The fact that you have a weapon switching system really emphasizes this, even if i'm not skilled enough to use it effectively.
However, there's a few things that bugs me, mainly the crystal we have to protect. It isn't necessarly a problem, but the endless stream of zombies attacking it are... There's this nice map with bumpers and cackodemons flying in the sky, and the player isn't allowed to go enjoy them because as soon as we leave the crystal, zombies start damaging it.
Not as important but the limitation on the shotgun's range seems a bit extreme, as is the dezoom of the camera when jumping (this is still an awesome idea, just needs to be tuned down a little in my opinion).
Overall this is a nice game, it feels really cool killing demons while jumping accross the map!