Really impressive presentation! The way the gun lights up, the way the bullet casings are heated when they come out of the rifle and then cool as they hit the ground, and also stick around forever! The dark, oppressive atmosphere! And the perfectly fitting music, of course!
In terms of gameplay, it's a bit difficult for me to say. I thought of a quote "Given the opportunity, players will optimize the fun out of a game." Because the camera is so close and the demons overwhelm quickly, I found the best strategy was to stand with my back to a dead end in front of a narrow passage and fire directly into the wall across from me, making all the bullets ricochet down the corridor and killing everything. As the waves went on my upgrades made this even easier, with reloads being so instant I never had to take my finger off the mouse button. Which is, surely, not the intended way to play the game!
Also, I noticed that when I restarted a run it carried over the upgrades I'd gotten, which was probably not intended?