Earth was destroyed in wave 12! 

I would have loved to have the option to add extra parameters in the equation from ground laser, so it could make possible to have "reversed" lasers from top left to bottom right (as in y = -1/1 * x + 10).

I guess this could imply that a laser ground to be launched from the "air" or even as an horizontal line (as in y = 0/1 * x + 5), so maybe not having this option was the right design decission!

Anyway, amazing idea and really really great job!! I really enjoyed playing the game and thinking about the right coordinates and linear equations!