Hm... The game have lamps, that make the terrain brighter. Maybe is possible
mess a bit with the code to create a invisible lamp that darkens the terrain
instead?

Like the "Impassable Tile", is invisible. Us can del with del_obstacle(). I just
wonder how could us adapt this code piece...

This appear a good idea, and I am also in favor of this. It's intersting,
however, as ahuillet said, it is not too much important right now.