The concept is very cool and the music is very calming. I did find the first cutscene play way too fast for me to read the text.
Also, you can use some more variety with enemies to keep the player interested. I liked the upgrade system and it provides some required variety in the gameplay.
Just before I started thinking that there is no more content to explore in-game, I came across the new type of enemies and the final fight so I think the game felt a bit repetitive until the fight. Overall, it's a good game but surely there is room for improvement :)