The art is cute, the alien scenario is great and the sfx fit the game but it gets a little boring for a number of reasons. The player and the aliens move slow and the bullets are slow. A quick way to fix this would be upping the timescale. Particle effects, loud sfx after killing enemies and camera shake are always big factors in classic 2d shooter games. As mentioned by others, bigger bg music would make the game more fun, something chiptune or alien-like. A little more challenge with the aliens would also be nice, such as more bullets shot or faster bullet speed.
Despite all my ranting, I do find potential in the idea of being able to use only certain areas that you put down for firing. It's like a mesh of tower defense and shooter games, two very fun genres for me. Nice work!