I did not like that I didn't know what I was supposed to be doing or, especially, how the health works and I eventually had to go to the comments to find the answer. Once I understood that the game became increasingly easy (putting aside the fact that you could theoretically farm the first wave of the first level indefinitely). I didn't even bother with paying attention to the blocks and just killed enemies until I was able to start unlocking the permanent perks, which then let me focus on the optimising the blocks and maximising a run. I also tried different kinds of wave perks and it didn't really matter, it just meant slight strategic changes to a level at times. I very quickly got to the end and maxed everything.
So, clearly it needs a lot of balancing and an explanation of the health in the beginning would be very helpful. However it's definitely a good prototype for a more complex game and more challenging enemies, plus perhaps wave perks that start to cancel each other out or work in opposition to each other so you have to strategise. It would also be interesting to have the grid expand or contract sideways in some sort of mechanic because you can't really use those end columns in the later levels. Also, and maybe this is a positive, you can actually use hearts, with certain perks, to help to clear low numbers off the grid so being hit can be advantageous if you know how to use it.
The music, while not bad, had a loop that was too short and so it got repetitive quickly but I like the art and overall feel.