I have a good idea.
I see the invisible tiles in the first level.
Why not make it impossible to see (IMPOSSIBLE) and add an easter egg after some impossible-invisible tile traveling?
http://img297.imageshack.us/img297/5729/tilezeastereggbi8.png
Sorry about my comment about higher then DTD. I made it seem like "WTF this game is WAAAAAAY overrated". It's not,it's awesome. But uh...
Did you get inspired from this?
http://www.emanueleferonato.com/2007/06/14/create-a-flash-ball-game-with-visual-from-above-tutorial/