Nice game. Sound FX and music were good quality, helped a lot with immersion, gives it that pro touch as well. I like the "collect money/gold/credits" and upgrade type games, but their all very similar other than the graphics that's laid over the top.
I think the game keep could benefit from some kind of combo system, to make it more interesting and add some kind of bonus system to earn gold faster, basically so you can be rewarded for skill. The intensity picked up pretty fast after the first level which was fine, but got swamped a few times and it was frustrating to not be able to move because I had an enemy bot standing over me, literally couldn't shoot or melee a couple of times in this situation.
The BGs and characters are nice, but I think at least with the BGs they are a bit sparse and could do with some more assets. Also maybe some distant BG images, like valleys, or destroyed cities, or the ocean. Plenty of options on your little island setting.
Really nice work, keep it up! I'd like to see one of these "earn and upgrade" taken to the next level - even if they are casual browser games.