For one. Your going to need to make this game look good to have a chance of getting any sales on steam. Right now it's obvious many of the visuals are prototype visuals. So I've got some tips.
1. You need to be able to aim your shot. Only being able to shoot left makes you camp the right side of the field.
2. Like I said the graphics will need a boost. A healthy amount of post processing and much more consistent and better art style will probably do the trick.
3. The variety needs to be increased. (But as a prototype it makes sense it's bland so no worries.)
4. Make the play field larger. The small arena makes it easy to die and also allows for minimal strategy.
5. Make a smoother difficulty curve. I could not get passed the first level of ten. Make it progressively harder not impossible.
6.Make an entity cap. (at the very least for the enemies) This would be a simple way to make it easier. By making so that only say, three enemies can be out at once you would make it much easier.
7. I would experiment a bit and see what your good at in game dev and emphasize that. This makes it so that at least something in the game has a good amount of quality.
8. Research the market. I know this is a little business-y but look around at other top down shooter games on steam and see what worked and what didn't. This makes it so that you make a game for an existing audience as well as seeing things not to do for your game. If you want to sell this on steam it has to look good and be good. So don't let yourself fall into pitfalls others have already fallen in.