I enjoyed the gameplay! I liked seeing the various enemy types and figuring out how to deal with them in different combinations. The addition of knockback to the player's weapon allows players to herd the enemy swarms in a direction, which makes this distinctive among other "twin stick" style shooters.
The eerie music, combined with the muffled sound effects and muted colors create an interesting atmosphere, like that of being in outer space or an extradimensional space.
I don't think the strange input for level select added much to the game. I would have preferred just being able to click the level title to select it instead of hitting 1-5 buttons to do the same thing.