It's interesting you bring up Sil'dih. I ran across an NPC in the conjurer's guild that spoke of a city beneath Gridania too. It was not Sil'dih that was referred to, but it seems they share a common trait.

Both appear to be ruins beneath these current cities.
Maybe there is something more to these?
I now wonder if anyone in Limsa speaks of a city buried under Limsa Lominsa.