Nice game. The only thing I could suggest is adding some way to unselect a piece you grabbed on to so you don't have to restart the level or use the wrong piece.
And for the Spartans, get the space laser and make sure to upgrade the ammo and radius all the way. You won't have any money left to upgrade anything else, but you won't need to.
For the zombies, I switched to teh shotgun and tried 3 times. I think i lasted 2:16 or something.
Also, I never interrupted the loading of the shotgun and let it go to 10 every time, so that might help.
At Nagoya, there's units with an insta-kill (like 3,000-4,000 damage move) that is used... hard to get it...
Good game though... a little easier than i would have hoped...