You have a solid mobile SHMUP here. The graphics are good and the music gets me in the mood to play. The current enemies and background are good, but get old. I'd suggest spicing things up with level progression and introducing new biomes with new enemies.

One issue I had with the game was the player shooting sound. It either needs to be toned down or done away with because it's simply annoying.

And my final suggestion would be to get an online scoreboard so I can compete. Here's my high score: