I think this is a pretty good game, this is one of the only physics games i know that provides a good challenge, level 20 was my max. Although what Dimono so eagerly said is true, you sometimes shoot 30 cannonballs just to get someone.
I think i did find a bug or so, at level 7 it doesn't matter wether the pink ones fall out of the map or not, you can simply shoot them :p