I like how the gameplay, although very simple, can get very intense and strategic, having to decide where to strike first. Graphics, sound and the "only one bullet can exist at a time" reminds me of some NES games, so this was also a nostalgia fuel for me. :)
One thing I might like to change is for enemies which take more than one hit to destroy to change their appearances after getting hit (perhaps to these of less resilient enemies) so that player instantly knows the situation just by looking at it, without needing to remember what was before, but that is just a minor thing and, other that that, it is a nice little game. Well done! :)