Hey ThatAmuzak,
This is an interesting approach to the tower defence mechanic.
I got a score of 359.1 seconds.
I liked the way you positioned the rings, it is quite pleasant to the eye. I also liked that you avoided using the colour red on anything other than the things directly relevant to the player.
The enemies were noticeable due to the movement but it would probably be better if they had a distinct colour as well.
The few pink trees on the right side did not really do much for the colour theme of the level.; if anything, they conflict with the red in turrets making the turrets slightly less noticeable.
One other thing that I noticed was the timer last digit went into a second line some times. (I think it was the milliseconds)
Other than that, it' a cool game. Nice work!