The art is great! The controls are tight (the wall jump mechanic works rather well).
The parallaxing background on the first level really needs some work. The speed it's currently going is pretty disorienting and it seems the movement of the background itself is tied to whether or not the movement key is being pressed rather than whether or not you are actually moving.
On top of the, because the background moved so quickly, accompanied by the color palette used, the enemy fire was extremely difficulty to see.
Sound would have made the experience overall much more enjoyable, especially serving as a tell for when the enemies attack.
Overall, though, I would have kept playing if it had been longer, so I'd say you're working with something pretty solid here!