This article contains plot details from the latest book.
The Green Door House, mentioned in Flashback, is a house with a green door, located in London (which is in the Forbidden Cities), that Keefe goes to pre-Keeper when his mom forced him to. This was discovered in a broken memory, not meant to be found, that Sophie and Fitz probed out of his mind. A possible ally of the Neverseen lives there. Keefe had to deliver a letter from Lady Gisela to the human living there.