Greenhouse
I don't have the books in front of me, so forgive me if I make a massive blunder. Anyway, I've always wondered about the ruin that Horn and Seawrack find on an island (blunder query: IS it an island?) It's an area marked off by a ruined circular wall of glass. The glass intensifies light in one direction. There are some kind of large or unusual trees or bushes in the area (I think I'm remembering this correctly.
What the heck is this? It sounds like a greenhouse to me. Somewhere where the Green Man's race was made? Something to do with the treelike Neighbors?
--Brian