The fort B is somewhere near Paris. It’s a part of a defense line which was build in late 19th in order to protect the city against intruders attacks.
At the beginning, the place was naked and the hills artifical, but now the trees have grown and everything is hidden by a wood, so that nobody can guess its presence.
You have to be very careful when you visit this place, because it’s currently used as training field by french special forces. In a few years it is going to be turned into residential place.