Displacement Sew works like this: You need to put the displacements edges face to face, you can be more precise with the Vertex Tool.
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Example:
If you put them face to face like this, and if they have different geometry, you can Sew them on the Face Edit Sheet > Displacements > Sew and the geometries will be reattached.
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Map runs smooth and is good, there's no need to any higher place to settle.
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-| THOSE ARE ALL MY VERY PERSONAL OPINIONS |-
There's no need to add platforms on the 4 walls surrounding the areas, you can try to:
-Edit their displacement geometry making them different in volume.
-Add some buildings on top of the walls background.
-Change brightness and ambient, or/and add falling snow ( falling snow is hard to make ? )
Note that mountains with perfectly perpendicular walls don't exist.
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