Love the art and the concept is out there! But I think you need some kind of visuals telling the player how far left or right they can go. Maybe like a bus that is dropping off the enemies? Also, I thought it was more of a side scroller progressing to the right, and not a back and forth - maybe tell the player visually with some arrows that flash at the start that they can go in either direction or have enemies already on the screen on the far sides of the start screen?