Not too bad, but...
I couldn't find a way to cancel tower placement (even clicking other tower types didn't do it), so the cost of the tower was gone even if the player change their mind or selected the wrong tower by mistake. You should change this mechanic so that the tower cost is only deducted AFTER the player places the tower, and allow them to cancel tower placement.
Also, some of the tower ranges seemed a little deceptive. Some towers either fired their initial shot very late, or didn't actually target an enemy within the displayed range until they got much closer.
Not a bad effort overall though! Tower defense games aren't the easiest to develop, but you did a good job.
P.S. Ninja turtles are awesome. :D It was cool when Ralph kept popping out.