Oh, I’m so glad someone asked this question.
Yes, oh, Hells yes. There are ruins abundant! Either it be new ruins on cities destroyed during The Infernal Incursion (Also know are the 5th Cataclysm) or ancient ruins that even pre-date the first Cataclysm that shaped the world as it is today. There are plenty of ruins to explore.
One of the most notable that I’m working on is located here (Image 1) the recently declared independent Kingdom of Corvengate. Originally part of Eldrathor, this Kingdom only recently declared its independence from Eldrathor as the Kingdom is in the process of rebuilding after The Infernal Incursion.
Corvengate, though similar to Eldrathor in turned of ecosystem, is also significantly different. As it is the best passage way to the North on the East side of Riftglen, Corvengate has been fought over countless times and houses countless graveyards, old battlefields, mausoleums and is hold to what many believe to be a massive ancient underground Necropolis.
Giant ancient statues of ancient unknown kings dot the densely forest land and mount sides. Old refurbished and repaired fortresses and tower guard old choke points and old caves are scattered everywhere. One reason people believe that there is one massive Necropolis super structure underground is some adventures have done quests to kill dangerous, man eating bears, tracked them to their dens and have accidentally stumbled into ancient ruins that resemble the many massive stone entrances that are know about that lead I to these Necropolis, where they are ment by hordes of undead.
There are also an ancient Dwarven deep roads system spans this worlds underdark and that accidentally breached into this giant tomb long ago, and has allowed the undead to spread deeper unground. Adding to the Dwarves struggle to reclaim their ancient underground highways.
(Image 2 is outdated but give you an idea of what Corvengate is)
alright as a long-time GM I have one. have you thought on any civilisations that predated the existing ones? were they underground? are their cities underwater when the coastlines changed? or are their ruins still about?