Who cares about showing "spoilers" if you want critique on the entire game itself?
So far, I have seen tiles, a boss, player ship, lasers, explosions, power ups, hud, cash, an enemy, and not much else.
To me, a spoiler is telling something that happens during the game. As in something to do with the story or what have you.
As critique artists we need to see as much stuff as possible. Unless you don't want any critique on what you don't want to show us.
As for me, I got nothing. Nice boss design by the way. Reminds me a little of homeworld.
Although, now that I'm thinking about it. You may want to resize your enemy ships. They look a little out of place and too clustered in compared to the powerful looking player ship. Now that I'm at it, might want to resize the player missles as well. I could barely tell that they were there until I zoomed in. And one more thing. Your tiles, I think, could benifit from a more speration between the shades. Instead of using a 10 point change, use a 15 or 20 shade difference.