Drow Question

Good Day Creatures!
I had a certain argument with my friend (who is a specialist in drow, as
he is famous for playing drow characters)about drow, which can as well
apply to other deep-dwelling races.
The question is: 
Drow cities are normally underground (according to my friend, sometimes
hundreds of meters deep). Meaning that they need to dig some sort of
shaft to allow spelljamming vessels to rise to the surface.
Now, should the shaft be straight vertical or should it be some other shape?
SY,
  MicroBalrog
P.S. How deep drow cities really are?