Yeah it´s really easy...if your are a noob and watching the walkthrough...i hate badges like that in this games there a walkthough button already is -.-
realy funny...i chose the theird stage, started it and saved it (without doing anything) . then i resume, build a tower, started the wave and no enemy comes. i end the game, choose new game, the same stage, the tower still stands their, but can´t shoot. and the funny thing was: i sold it, but it doesnt go away...so i sold it, sold it, sold it and i was rich...
well they shouldnt give so much points for "completing the game", because theirs a walkthrough link, and it´s just easy, to click on "need a hint?" and watch the answer -.-
What´s your problem? I mean, this poor sweet blue box trapped in this box, and now it´s FREE!!! isn´t that a happy end?? (well, but I should stand in the box, cause its look better than outsied ._.)