The Green Room is located in Van Nuys, California5.18 Point Of No Return It's a place where angels can manage reality. Zachariah and Castiel use this room to secure Dean Winchester. Zachariah later uses this room to secure Adam Milligan.
Zachariah refers to it as a "Green Room" - which is the term used for a room where performers wait before going on stage. Dean also refers to it as "the beautiful room".5.18 Point of No Return The room visually references the room in which Dave wakes up in the final segment of the movie 2001: A Space Odyssey which is called "Jupiter and The Infinite Beyond" and which represents a realm beyond the mortal.
Zachariah takes Dean here as he is waiting for Sam to kill Lilith, and therefore free Lucifer. His plan is that Sam will be taken as Lucifer's Vessel, and the Michael will use Dean as his and kill Lucifer.
The Green Room resembles a luxury hotel suite. Zachariah and Castiel can cause doors and windows to appear and disappear, and when Dean tries to break through the wall, it is instantly repaired. When Dean first enters the green room, the paintings on the wall are bright, pastoral scenes. Later when Dean confronts Zachariah, they are dark surreal scenes of Hell and battle.
Zachariah takes Adam Milligan here as bait for Dean. When Castiel brings Sam and Dean here to rescue Adam, it is discovered that the Green Room is located in an abandoned muffler factory in Van Nuys, California. It is here that Dean kills Zachariah. At the end of the episode, after Adam has been taken by Michael, the room reverts to an abandoned office.