Every doorway leads to another
possibility. Another
future and timeline. The realities they lead to are exactly the same as the waking world cosmology; they would need to be to account for the infinite
multiversal travel and possibilities that would lead to interacting with spaces like
Limbo and the
infinite amount of worlds. Since entering a doorway is a possibility, clearly they would also be included in the larger Hallway itself as simply another future. High-tiers can affect entire dreamscapes. When they do this, they are affecting the Infinite Hallways as a whole, so it would also be affecting the Hallways within Hallways within Hallways. Dreamscapes would contain all of these as a whole, and thus would be considered a Low-1C space, that they are affecting with their own power. That's the entire point. They are all within the dreams that Luna and Celestia can affect - and they
affect many thousands, to possibly millions of these at once by the way.