Click & Drag towers to move them

Projectiles should pierce through multiple enemies before they stop. They will aim for the furthest along enemy that is also close by (so they can go across to a different part of the track and can go to enemies that weren't inside their tower's range).
Each tower has a range of 200 pixels (that's the radius of its range circle) and it will target the enemy that is both within its range and is furthest along the path.