Reference to "The City of 9 Gates" can be found in an Ancient writing called;
"The Acts of Peter and the 12 Apostles".
(Another Ancient "Parable".)
It can be found on the Net.
Sadly a more recent Translation though :(
Link....
http://www.gnosis.org/naghamm/actp.html
But still well worth reading.
Also reference is made to "The City of Habitation" (Our Source, outside this Little Universe)
And "The City of Endurance" (this Universe)