The layer should not be used to stack buildings, the buildings should be on the same layer. Except for underground ones, you can use layer=-1, so I set the building base as the main body of the building. 
And the other hand, "building intersection" problems will occur. 
layer應不是用來疊建築，建築應該在同一layer。除了地下的可以用layer=-1，所以我將建築物底座設為建築物主體
不然會發生"building intersection"的問題。