Citation for the rear gate and its stairs:
A broad stairway, climbed from the Deep up to the Rock and the rear-gate of the Hornburg.
The Two Towers, Chapter VII
EDIT: Oh, OK, you were talking about that thin Dunharrow-like thing. I didn’t even saw it. I understand better why you were talking about a plateau now.
I just found out the map is clearly inspired by Fondstadt’s, btw: