Man, I'm BAD at TD games. After 5+ tries, I've never gotten past level 25 on normal. -_- I make them zigzag back and forth for the entire board, but clearly this isn't enough...
@WallieTheKing, thanks, I got it by dumb luck one time. Suggestion: Pressing Enter should confirm what is on the screen rather than assuming "Continue" - I had clicked "Restart" and then instead of clicking "Yes" I tried pressing enter - didn't work so well. Also, the hint says you can "press Enter or Space to Restart - Try it now!" Space does nothing, and Enter does continue only (unless you're out of continues, at which point it restarts). Maybe have Space be restart, and Enter be continue?
At first I thought "% Loot" was the ratio of loot picked up to loot seen on screen. Then I realized "achievements" would have been a more accurate term, and I relaxed.
I spent a couple of hours getting perfect on the first 7 levels, thinking it might be required to unlock the bonus. Then I said "screw it" and lowered my standards. I am quite glad that I did. :)
Man, I wish I'd known that you can get a perfect, even if you lose hearts (as long as you don't lose a ship). That could have saved me a LOT of restarts. Glad I found that out on level 6, rather than a lot later on!