The best thing about the game is that you can RESIZE it for you screen. Very nice. Better than CTRL + "+".
No interest though. It's too hard to get money, so you never get to upgrade or level up much. Rewarding frugality would be a nice touch.
I find that you'll be doing well (played 3 games) then some level will just blow by you unscathed.
But as TD games go, it's quite good.
Now, I'm not playing anymore! Too addictive!