We can see different strata of their history in the buildings that exist, although forerunner architecture is still standing, almost none of the middle age structures still exist, and if they do they are in varying states of decay.
Aether-based weapons, global holocaust.
Several nations rise and fell, kings eventually become democracies, and The Republic of Wayland was formed during this renaissance based on an idealized relationship between predator and prey. During the Rosewater Renaissance the inhabitants of this world discovered human history and (since their own history was mostly wiped out during the calamity, along with many species) they rebuilt themselves in humanity's image. During the apex of the modern era the Wayland's technological capabilities were around the United States during WWI. But the rot has continued to accelerate, leading to the post-modern era.
At this point many great societies have suffered dissoution as the inhabitants are unable to maintain their basic social services or their economy. Religion has become an important part of every life although the previous inhabitants did not really believe in an afterlife. They have lost their ability to walk on two legs. At the start of the book The Republic of Wayland is the last functioning government. Society as a whole is on the verge of collapse as different factions desperately fight over control of remaining resources.
Strange glitches are appearing in reality like the Deadwood or Apocalypse Land, as if fundamental aspects of reality like space and time are succumbing to the Rot.