Nice work, but needs work
I really do like this game; it's probably the best of its kind I've ever played. It has compelling challenges, interesting customization options, and some satisfying moments. Yet I can't feel like there's too much missing to call this a great game.
Really, all of my complaints lie in the 'challenge' system. Challenges should, by nature, be challenging, not completely impossible. EARN $10,000,000?? KILL 5000 HENCHLINGS?? I've been playing for a fair amount of time and I'm nowhere even NEAR those numbers. And the rewards aren't even that good for such insane accomplishments (using the word loosely) either. Even the sniper badge is probably harder than it should be in my opinion. And why do I need to do something as difficult as go through the first five waves without a hit to unlock something as integral as the selection screen? The way it should work (not that I want to tell you how to do your job...obviously you're a better programmer than I am) but the way it should work is that different achievements unlock different characters. Not having all of them grouped into one, unnecessarily difficult achievement.
Also, none of the challenges extend past level 5! Once I hit this level, I was really not compelled to go on anymore. IF you put challenges in a game, you have to expect that people are going to measure their success off of the awards they've received. After hitting wave 5, part of me feels like I'm done. This feeling intensified once I go the diamond boat. Why keep going on after this? And why get rid of the upgrade screen? I need a brake every once in a while even after I'm fully upgraded. Why don't you allow the player to buy health between levels at this point? Or extra characters? Or new mine types? You need more cash sinks.
Sorry to rant, but I feel like there's so much potential in this game and yet I barely want to continue playing it. I feel like you can do better, and will do better. In my opinion, this project is worth continuing to a sequel. Maybe new arenas, enemies, weapons, statistics, characters, character storylines, powerups...all of these things could make a really awesome game. I hope some of these suggestions are helpful.