First of all, I really like the art style. It reminds me of what Gameboy games looked like.
I also like the little explosions when you hit an enemy. It indicates to me, that I'm hitting the right target. For example the little health pickups don't register a hit, so I knew I could pick them up.

What I didn't like was the lack of any help.
- When I started the game, I tried to move with the WASD keys, because I expect to move with those on a PC game. I think it's fine that I have to use the arrow keys, but a little heads up would've been nice. This is of course not an issue with gamepad controls.
- Next, I had to figure out how to shoot. Because the game uses the arrow keys for moving, I had no idea which keys I could press and tried out the mouse buttons. Again, this is fine, but I don't like being thrown right into the game like that.
- There is no pause button (that I'm aware of). I was trying to pause the game to write notes and couldn't find a pause menu. I tried the escape button and everything on my mouse and around WASD.
- The game doesn't have any start or end. When you finish the boss, you get thrown right back to the start. I think this is fine if intended.

Bugs I encountered:
- The boss had a bug on my first run where the center gun and the top and bottom guns fired simultaneously, which led me to die instantly.
- When shooting, the space ship moves to the center of the screen. When this happens, it kind of bugs the movement controls and you have to let go of the move keys to start moving again. If I didn't I couldn't move all the way up, for example.

What to improve:
- Add a start menu to the game. It doesn't force me right into the game and I have the ability to look up the game's controls
- Add a pause menu. The pause menu doesn't have to be super fancy. Imo, it's enough to show a "Pause" text in the center of the screen.
- Either in the start menu, the pause menu or at the start of the game: show the player how to move. A player won't read the description on how to play before trying to play.