Thank you for your kind words, I am glad you enjoyed it so much! :)
Being able to shoot enemies was more of a side-effect of my exhaustion. I wanted to make some enemies shoot something at you, but I felt too lazy to program a new kind of object. So these bullets are just a type of an enemy ship (spawned every 32 frames by each shooting type enemy ship) and, as such, they could be shoot at by default, so I though why not use that to enrich the gameplay. :D