[ainur]

i guess I always thought that Shelob had her own tunnels and that the men of Gondor built the stairs and their tunnels themselves when they built Minas Ithil. Shelob then opened a passage into them, perhaps centuries later after the Minas Ithil became Minas Morgul. I can't verify any of it though. I know of no specific mention of her whenever Minas Morgul is discussed except from Gollum's plottings. It's possible the connections between the two sets of tunnels was not made until around the time of "The Hobbit" when Sauron was driven from Dol Guldur and returned to Mordor. But like I said, I can't verify it. Just speculating.