Really cool idea mixing artillery and tower defense!
Love the pixel art, too.
Most of my shots were very rapid shots close to the portal forming a "cascade" of magic because I wasn't able to stop the enemies otherwise. That's probably not the intended way to play it, but I was too slow try to aim without success. xD
The window was too big for my monitor at first and had to resize it.
Also, at some point the sound just stopped for unknown reasons.