Ul'dah was moved above Sil'dih's ruins to take advantage of the aqueducts and the superior irrigation. This bit them in the arse towards the end of the Thorne Dynasty, when Ashkin rose up from the ruins and Ul'dah spent years beating them back.

If Sil'dih is anything like Belah'dia or modern city-states, they'd have sets of ruins beyond the borders of the main Sil'dih itself. Were modern Ul'dah to fall into ruin, you'd likely find ruins all over Thanalan - from Drybone to Black Brush to Horizon. Sil'dih may have been the same and had camps and settlements beyond the main city.

I've brought this up to Moose as a possibility before, but...



Given what we know of Ul'dah's history, is it not possible that this mirage is not of Sil'dih, but of the original city-state of Ul'dah prior to its relocation if not, as Moose said, a simple urban legend?