I was watching this video.
At time 12:20 into the video he explains about the building process.
I’m not clear if I understood this correctly but I think he says that the walls was put there after placing the wall decoration mesh.
So if I get this correctly you build the walls last and not first for example.
Could one see it as it is recommended to build inside to out? Start at the center add all meshes and work your way out and add the walls last.
As far as I understood it, you place a simple shaped wall first - then you add the decorations meshes + turn the basic shape into a collision. 
Like this:
blue = the basic wall shape (will be converted to a collision)
red = the decoration mesh
