Space and Shadow

Suppose a (Arch?)-Master of Space were to create a rather extended sanctum/ Demesne using some combination of Labyrinth and Hide Space, making the space inside of the sanctum rather larger than the surrounding space.  Effectively, the Master builds a castle under a rock.

Now a Thyrsus wanders into this place and decides to make a Spirit Road.  Where does the spirit road lead?  Is the Shadow warped to create a Shadow reflection of the castle, or does the Spirit Road dump out into the 'normal' place behind the Gauntlet?

Probably the second one. Not everywhere in the Shadow corresponds directly to the Material Realm; buildings can be larger or smaller, cities can be overtaken by forests, hills, or coasts, and there are plenty of places with no actual physical presence, such as prehistoric or purely elemental environs. Similarly, a place in the Material Realm might not have a direct spirit refelction, particularly if it is a place that few people interact with.

I'd say the latter.

ditto on what everyone else has said.