Quote from Nethaera, Blizzard Community Manager
As a side note, once you do this you will be able to build your buildings ANYWHERE. You will probably want to flag "Ground - No Spawn" flag under the "Invalid" flags, as well. 
This will prevent you from building your buildings on other buildings (no matter how much Xzibit may want you to do so), and on cliffs.
This will prevent you from building your buildings on other buildings (no matter how much Xzibit may want you to do so), and on cliffs.
Link to the thread
None.
