Speedo, every game must come from somewhere. Gemcraft has "essence" of Desktop Tower Defence. Call of Duty has "essence" of Medal of Honor. As long as you don't full on copy it, it's ok.
This game is wayyy underated. It's not just a game made because it has an easy badge. Play for a while and you will get hooked, I assure you. I'm tired of seeing everyone stop playing games because they have the badges.
Pricing your games. Dude. If you made the full version free, you would get 10x as much cash because this is a great game and the advertising and plays... 1/5 for demo, but 5/5 if it way full.