The shadow City is a secret city beneath Manhattan. The Irregulars explore and observe it. For it is full of rats you should prepare yourself before entering.
In 19th century, the shadow city fluorished. It was a place to hide for villians, wich you could enter through nearly every place of ill reputation in Manhattan. In 1866 Percy Leake III. wrote a guidebook to the darker sides of Manhattan, where it is described as following
"Police raids are common in the more colorful parts of town, and gentlemen explorers may be mistaken for criminals. However, if in the midst of your adventures, you find yourself in a bit of a spot, do not despair. Simply ask for the way to the Shadow City almost every place of ill reputation the isle of Manhattan will have an entrance, a network of tunnels, that can serve as handy escape route,"when things get hairy". And if you are not disturbed by the thought of the countless criminals who make it their home, the Shadow City also offers an ecxellent means of getting about, when the weather aboveground is unpleasant. Be forwarned. The tunnels of shadow City are uncharted territory, and anyone willing to give you directions is likely to lead you astray. Many have wandered for days without finding a suitable exit to the world above. Others have never escaped."
Even though, the tunnels of the Shadow City were also used by the "good side", as for saving slaves, wich Ananka writes an essay about in book two.
After a plague, many people died inside the Shadow City and it was left and then forgotten for a long time. Kiki Strike and the Irregulars rediscovered it and made a map. It is full of rats, and since it is perfect for commiting crimes (hiding smuggled goods or breaking, for example, into Chinatown Savings and Loans, a bank in China Town) the Irregulars defend it from criminals, especially the Fu Tsang.