Very fluid movement and really great background music, sound effects and special effects.
I am not a fan of the aiming. Monsters can come at me from both sides and sometimes I have to drag the crosshair across the screen and back. Also, having to control the crosshair both horizontally and vertically sounds a bit too excessive in a side scrolling game where your movement direction is pretty much along just one axis. I would recommend taking input from the mouse's horizontal axis and determining the angle of shooting depending on that.
This is my only real complaint but if you are planning on an update, I would also recommend adding some tile variation, and multiple difficulty levels.