Just a couple of bugs I noticed, the bullet sound seemed to have cut out after wave three. I'd also have liked some visual for the bullets, since it was kinda hard to tell if my shots were landing aside from the enemies disappearing. The health was also a little weird as there was no way to tell if I got hurt until I died.
I liked the random spawning location for the enemies, and I had fun trying to survive. Maybe next time add some more visual cues so the player knows what's happening at a glance, ie the enemies flashing red or something. Good job overall :)
P.S. WASD tends to work better for these kind of games I think, but I suppose that's personal preference