Very cool hand-drawn art style! And fighting off hordes of alien monsters was frantic fun. I especially liked the lizard/salamander-looking one, they almost look too cute to shoot :)
I like the fact that you included multiple different weapons, and that they all feel different. It was a bit confusing at first what the bullets were and why there were additional particles being emitted from the gun, but that's a minor point. The particle effects of the explosions looked really nice. I like the music, fitted well to the setting of the game. Also the 'endless' aspect of the game is really cool but maybe giving the player a bit of reprieve in-between waves of spawning might have been good (but less chaotic, I know). Eventually I was overwhelmed by enemies because they were spawning faster than I could take them down and they cornered me I a place where I couldn't get to the gun that had spawned.
One technical issue I had was that the game didn't display properly on a widescreen monitor (the computers continue button was cut off) so I had to use my 2nd monitor instead. I think the enemies need proper pathfinding, they tend do get stuck on the level geometry, (presumably because they are always walking straight at the player?). Also the restart button doesn't seem to work.
These are only minor issues though and I really enjoyed the game!