I was trying right click and left click to place towers on all of the tower options but I wasn't able to do anything for a while which was a little bit frustrating. After a bit it just started working, but it was kind of frustrating not having been able to place any towers because the waves automatically start.
Maybe instead of auto starting waves, the theme "countdown" could have been integrated in different ways, or you could make the timer much longer and give the player the option to skip once they're ready.
The music didn't really fit too well in my opinion, but I liked the art.
I think this was a really unique type of game, and I think it has a lot of potential. Making this in 4 days is still really impressive, so good job either way! (Last thing, maybe using WSAD for moving the camera would be nicer than using mouse to move it?)