From the introduction:
When the underworld sea drained away, it left a vast depression full of unbreathable air - the Ur-Menig. Near its center is Sifoon, City of the Carreg.
Once a great trading hub, the dwindling Carreg have been unable to hold Sifoon's districts against the endless tide of external and internal enemies.
For years, their hold over the city has been only a few neighbourhoods near the Port.