Use the arrow keys to move and space bar to shoot. You can also use the WASD to move and mouse (left click) to shoot. I find it easier to move with WASD and mouse, because your space shooter character follows the direction of your mouse, so it's easier to turn around.
Sorry that the health bars go in front of everything when you die or when you start playing. I will hopefully add a score, but last time I tried, I couldn't shoot and the Limes wouldn't start invading.
