A lot of fun!
This was a good, original take on this kind of game. A really awesome premise, and well put together. Overall, it was an excellent, and FUN way to incorporate platforming and TD gameplay.
Definitely sequel-worthy, but I don't think that adding a buttload of new upgrades and stuff is really what you want.
It seemed as though the waves topped out at one point... with my turrets all upgraded and the upgraded machinegun, suddenly none of the enemies were a threat any more. Maybe more and bigger enemies would be a good answer. It would keep the player on his toes throughout the game, not just at the beginning.
Also, the mechanic where you lose accuracy as you get more tired makes sense -- but it means very little once you get the machinegun, and the enemies are all clustered together anyway. Maybe your fatigue should affect your rate of fire/balloon repair as well?
Lastly, the most expensive gun, even fully upgraded, is not nearly as awesome as the cheaper machinegun.
Really great job, you guys. I thought the music choice was spot-on, and you're keeping it classy with the Mark Twain. Keep up the good work.