The sewers constitute a vast network of aqueducts and tunnels that spans the entirety of Dunwall. Initially, they were connected to canals as a place for overflow to drain from the Wrenhaven River to prevent flooding. Later, aristocrats found it fashionable to have passages built underneath the city that were attached to their estates. As Dunwall advanced, the individual sewers became connected to each other, sometimes intentionally and other times, accidentally, creating the current Dunwall Sewer System.
As time passed, some of the older tunnels began to collapse, creating giant sinkholes. However, now this is no longer a widespread problem, as during the reign of Emperor Eurhorn Kaldwin I, the sewers were reinforced and expanded.
Corvo Attano uses the sewers to escape from Coldridge Prison during the Dishonored mission. He will also briefly visit the sewers beneath the Hound Pits Pub to deal with a pair of weepers prior to the House of Pleasure mission.
During the Flooded District mission, Corvo uses the sewers to travel from the Flooded District to the Hound Pits. If he has done any missions for either Granny Rags or Slackjaw, his path is obstructed by a locked gate. Granny Rags holds the key, and can be found deeper in the sewers, along with the imprisoned Slackjaw, unless Slackjaw was killed in the mission The House of Pleasure, in which case the key will be on Slackjaw's corpse in Granny Rags' hovel.
- Some of the City Watch use the sewers as a means of disposing of dead bodies, assuming the rats will deal with them.
- During the Dishonored mission, there is a safe in the sewers with a note next to it. The combination for the safe is 451, and can be found behind a row of whiskey bottles shelved nearby.
- Two City Watch Guards encountered during the Dishonored mission note that many plague victims have committed suicide by drowning in the sewers.
- The sewers were used by Morley rebels during the Morley Insurrection to gain access to important locations and mount assassination attempts.
- Hiram Burrows is rumored to have informants who can navigate the sewers quite well.
- One of the systems that was eventually connected to become part of the Dunwall Sewers is the Drapers Ward Sewer System.