I particularly like how frontal assault stops working, you start having to get tactical with it once you're past maybe halfway through. and the outside battles after you've "won" ... well, basically, getting all the achievements is just completing the tutorial. The real game happens after.
Damn. I remember playing this game long ago. It was lovely. Now it gets laggy, not what you want in a paper-airplane-flying game. Obviously not the game's fault, too bad.