Use the arrow keys to move back and forth (not the ones with the numbers), and space bar to fire.
You can fire if you still have a shot on the screen, but if you do, your old shot will disappear.
The aliens go back to the top of the screen after coming down near the bottom. Some people have reacted negatively toward that. I am not sure if I will change it, I am leaning toward giving a bonus if they are all killed before any go back up to the top.

Please send me any feedback you might have Abas@tmbg.org.