“Over the land there lies a long shadow, westward-reaching wings of darkness. The Tower trembles; to the tomb of kings doom approaches. The Dead awaken; for the hour is come for the Oathbreakers: at the Stone of Erech, they shall stand again and hear there a horn in the hills ringing. Whose shall the horn be? Who shall call them from the grey twilight, the forgotten people? The heir of him to whom the oath they swore. From the North shall he come, need shall drive him: he shall pass the Door to the Paths of the Dead.“—Malbeth the Seer (Lord of the rings)
The city of shadows is the entrance to the world of dead, half-forgotten ancestors.
The city’s image was influenced by the works of Howard Lovecraft (“The Shadow over Innsmouth”), descriptions of the ancient cities of the Númenoreans in the Lord of the Rings and the paintings of Hubert Robert. Robert was mostly famous for romanticized images of ancient ruins.