Hey, just got around to playing your game and I think its great! I enjoyed playing it for the time that I did, but i definitely think it could be expanded upon. These are just a few problems I had with it:
-I didn't find it too hard.
there was definitely big waves of them coming at me sometimes but I could just hold down the mouse button and id get away pretty good, considering i run faster than the enemies. I think a few way to bump up difficulty is to have some kind of ammo so that the player has to reload if they just hold down the mouse button and don't properly aim. and another way is to maybe lower the players walking speed just a tiny bit.
-music stopped after a while
Not sure if intentional, but since I survived for so long, I ended up just not having the music play anymore. It wasn't that big of a problem, just thought I'd mention.
-no way to close game
i mean, i just alt f4'd out but there was no way to go back to the menu so yeah. not too big of an issue.
I of course enjoyed the game, those were just a few problems and i think you could easily fix them if you were to continue development on this game and i do think you should. Anyway, great game :D