Shadows behind walls (fog of war)

I have a game where I need shadows (fog of war) behind walls. It´s not just a fog of war beginning at a specific distance, it´s a fog of war behind walls similiar to the one in this game: http://www.youtube.com/watch?v=z_YytQWZjAQ

You can see a prototype of the game here:

The shadow or fog behind the walls should change every update when the player moves. I read about projectors, but I am not quite sure if they can fulfill this task.

Another idea was to instantiate "fog-meshes" on top of the terrain with changing vertices every update. The Terrain will also be even in the final version. So this could maybe work. Does anyone have an idea how to cleverly achieve my goal?

Note: I am using Unity Free in a student project, so the real-time-shadows of Unity Pro or Tools like Fast Shadows (Asset Store) are not an option.