The music and the concept are amazing, had much fun to play the game and i really liked that you can uprade your skills.
The only thing, but i think thats something more personal, is that all the little aliens respawn when you go to your ship... my struggle was that i didn't made it so far as i wanted to collect all items the enemys drop, so i was litterally mostly of the beginning on the map and killing aliens just like a psycho.... As the oxygen goes down really fast i ended up killing the first 4 aliens, rushing back to the ship and do the exact same thing...
Overall i really liked the music the artstyle and especially the gameplay (except from my personal issue) Great job!!