DotA? I’m afraid you’ve got a wrong impression, because we are talking about some real Ancients.
A lot of towers are in the first quadrant of rectangular coordinates. And between every two towers there is a wall. All of them are Ancients. Somehow, some skytroopers are trying to attack our Ancients and all we know is that they’ll land at the second, third and fourth quadrant of rectangular coordinates. Because those areas are desolation, the troops decided to follow a fixed straight line. Now we’ve got all the landing coordinates of the troops and the polar angle of their walking lines. To protect our Ancients, we need to know where the troops will first arrive at the Ancients.