Cool concept and I like the art, but goddamn the audio is horrible.
The gameplay lacks a greater purpose and it doesn't matter if Earth explodes since I can just reset and keep all my upgrades, it could play like a typical wave defender kind of game where it gets harder each level and you lose all progress when you die.
There are also some usability problems, I wish I could see how much money I have without pausing the game and I can't tell which ship has each weapon, it would also be nice to able to shoot each weapon individually too. The buy menu is kinda confusing too and could have better art.
This could be a very solid game with some extra balance and polish, great work!