How to find a biggest gap between objects?

The path you are trying to find is neither the closest distance between towers beginning with the closest tower or the farthest distance. Maybe you want some point along the current path. Without counting for the current path/direction it is not possible to find the line you want, because it could just as well be straight down in your example.

Here is a method that might help you, but i'm not sure.

 static public float PointDistanceToLineSeg(Vector2 p, Vector2 l0, Vector2 l1/*, out Vector2 o_vClosest*/)
 {
     Vector2 l0_l1;
     float dist;
     l0_l1 = l1 - l0;
     float l2 = (l0_l1.x * l0_l1.x + l0_l1.y * l0_l1.y); // i.e. |l0-l1|^2 -  avoid a sqrt
     if (l2 < 0.01)
     {
         //just take l0, the line is too short
         dist = PointDistance(l0, p);
         //if (o_vClosest!=null) o_vClosest = l0;
     }
     else
     {
         //consider the line extending the segment, parameterized as l0 + t (l1 - l0).
         //we find projection of point p onto the line. 
         //it falls where t = [(p-l0) . (l1-l0)] / |l1-l0|^2
         float t = Vector2.Dot(p - l0, l0_l1) / l2;
         if (t < 0.0)
         {
             //beyond the 'l0' end of the segment
             dist = PointDistance(l0, p);
             //if (o_vClosest!=null) o_vClosest = l0;
         }
         else if (t > 1.0f)
         {
             //beyond the 'l1' end of the segment
             dist = PointDistance(l1, p);
             //if (o_vClosest!=null) o_vClosest = l1;
         }
         else
         {
             //projection falls on the segment
             Vector2 projection = l0 + (l0_l1 * t);
             dist = PointDistance(p, projection);
             //if (o_vClosest!=null) o_vClosest = projection;
         }
     }
     return dist;
 }