Great lovely game !
It has still some points to improve. It would be great to have a checkpoint system, I died at lvl 6 and I restarted the whole game ! I would appreciate a feedback when I got hit, for example a sound and a little animation, to let me know that I got hit, as well as a short immunity time after getting struck. Sometimes I lose 4 lives because I'm getting pushed from the back from a single monster and due to the fact that I can't shoot left, I can't avoid losing these lives. And last, gravity is well done but I think restraining the gameplay too much. For example it's nearly impossible not to lose a life when I jump in front of an UFO without firing 3 bullets.
Otherwise great job !