At the very beginning of The Return of the King (film) Frodo and Sam are awakened by Smeagol/Gollum after sleeping in a diamond-shaped stone structure of some kind. I feel like this place has a name, but it's been a while since I've read the books. Reading this online script of the movie, it is only referred to (Scene 2, Journey to the Cross-roads) as a "concrete culvert".
Does this place have a name? Who likely built it?