Every 13 years, a metropolis wakes with the buzzing sound of life. Do not cross its borders uninvited, especially not when it and its inhabitants are at rest.
The Sleeping City is a futuristic (1920s-era) city filled with jazz, high-rise buildings, and a whoooooole lot of unspecified bugs. It is aligned with The Structure.
Somewhere between two mile markers, there is a space much less than a mile, and within that a mysterious city sleeps until every thirteenth year, when it opens into normal space. It is slated to do so some time this year.
Duvall spent much of his inheritance on an expedition to find the Sleeping City, and he did make it there! ... but at the wrong time, so all he got was some of its guardians as his new friends.
Culture
Some of Davia Pledge's music cylinders appear to resemble in style the music of the Sleeping City, but it is unclear if there is any direct connection. So does the single record bought by the Toll Collectors.
Currency
Instead of a barter economy, the Sleeping City has a centralized currency with both coins and banknotes, and a cohesive art direction.
Passage on the Jade Moon arc
Duvall incorporated some of the power of the Sleeping City into the Perpetual Oratorio by leaving a few bugs in Virtue's cocoon. In thanks, it sent him tickets for the Jade Moon, a luxury steamboat to provide him and his companions with safe and enjoyable passage back north.