Woah, this is actually hard. Despite several attempts, I was not able to finish it! (the last attempt was really close though)
Nevertheless, this is a neat game! It's very rewarding, has a lot of juicy feedback, and overall is fun to play. I like that the hero himself also has its own upgrades and power-up. It's a cool detail, and even allows some strategies. The only trouble I had at the very beginning was failing to understand that I could not spawn monster inside the circle (I thought it was the contrary). But once I got it, the game was really intuitive.
I like the art and sound-design too. One little regret though: too often you can see the edges of the map. :P Feels kinda weird, but easy to fix anyway. Overall it was neat and pleasant!