You have to gain level. But you only can save your level if you play NASTY at nonoba.com.
@sysy: I don't know why, but some people can't place towers because of a bug. But as long as I can't see the bug myself I can't fix it :(
It uses the Nonoba API so highscore and level only works for Nonoba.com Users. A small description of towers is on the bottom right of the battle screen.
Sorry.. I am trying to fix it at the moment.
Working version:
http://nonoba.com/diadas/gravity-ball-multiplayer
It's an API from Nonoba so it doesn't let me upload it to other websites.:S