What a coincidence that we made similarly styled games!
I like it, there is a variety of enemies and it feels good to acquire upgrades. It is impressive that you have used C++. I like the music.
My only criticism is that it could be better tuned. I found often that a very large wave of enemies would appear suddenly and I would be destroyed without a chance to fight back. If it wasn't for this, and with a bit more balancing, this could be my favourite that I've played so far gameplay-wise.