Like a picture postcard of a vacation, when humanity began to put more faith in cold hard facts (and cold hard iron) over whim and fancy, the mythic parts of the world branched off to an overlapping level of reality called The Dreaming. Not quite Arcadia, it fit more with what both Fae and non-Fae felt that fairy-land should be.
Initially, this may have served as a buffer zone between Arcadia and the mundane world during the Sundering, connected to both by way of trods. When the Shattering took place, all paths to Arcadia itself were closed off and only the very rare path to the Dreaming remained. Fortunately this still allowed a slow trickle of Glamour to reach the mundane world. Only during the Resurgence were the trods between the Dreaming and the mundane world re-opened.
The Dreaming is better called "The Dreamings" (plural) since it is divided into three layers: The Near Dreaming, the Far Dreaming, and the Deep Dreaming.
Near Dreaming
The first is the closest to the mundane world and often reflects it, albeit in a more fanciful way; a factory might be expressed as a huge clockwork mechanism, a bustling mall as an open-air forum, an airport as a landing pad for dragons and air-ships. When in the Near Dreaming, a changeling finds their fae mien more 'real' than their mortal seeming in the reverse of how things are normally (where the mortal mien has precedence and the fae seeming almost seems like a daydream). In Freudian terms, this would be the dreams of the Superego.
Far Dreaming
The Far Dreaming is more the dreams of the Ego, with little matching landscape to the mundane world. Not only is the fae mien of a Changeling more 'real' here, but it is almost a caricature of how it normally would be (for better or for worse).
Deep Dreaming
The Deep Dreaming is more the dreams of the Id and is largely defined by the opinions of the viewer. For this reason, few ever return from the Deep Dreaming due to a false exit appearing to lead one to a false destination when one wishes to leave.