Half a King question

I was thinking the same thing. I'd have to do a re-read to pick up on any mentions of what ended the "old world." I follow Abercrombie's blog and I don't recall him making any mention of this on there or any interviews.

Yes. It felt a little like the Broken Empire world (i.e. Mark Lawrence's Prince of Thorns) to me. I imagined a post-apocalyptic world in a region like northern Europe that reverted to a Viking-like culture. Just me...

The signs with arrows & elf writing sticking out of the river as they go through the elf city on the boat. Bullet casings at the end of the fight with the horse people?

It becomes very much clearer in the second and third books that yes this is post apocalyptic, possibly some sort of nuclear or biological war (hence the sickness if anybody goes near the ruins)

Abercrombie seems to come back to the theme of ancient architecture that the people in his book cannot understand. The First Law series has a building (The Tower of the Maker?) that no one can enter or understand.

Actually, I kind of like that sad trope in modern fantasy of all the magic/ dragons/ artifacts leaving the world. It goes back at least as far as Lord of the Rings, right, when the elves are all leaving Middle Earth?