I had a lot of fun with this. Played through the whole demo.
In terms of difficulty, I think its a bit on the easy side. I was getting mostly 3 stars without really trying, and at the very least two stars. I think you give too much gold for upgrades. Its very easy to upgrade early, and upgrades are very OP.
For example, the bomb and fire are lethal, and once you have those (quite early on), you dont even need to upgrade anything else. I bought everything and finished the demo with just under 4000 gold to spare.
The hook is such an awesome weapon. Really enjoyed using that on the COAST levels.
The COAST levels are marked as HARD, but Im not sure I would agree with that. They didn't seem particularly hard at all.
I quite liked that you gave challenges too on a lot of the levels. That was a nice touch. I went back and played a few levels to see if I could complete them while completing the challenges. I can see that being fun.
Overall, a fun game. Enjoyed playing it. It was a fun way to spend 30 mins.