In City of Bones we got to visit the Bone City, aka the Silent City. It's a creepy place made of bones—what, you were expecting gingerbread?—and watched over by the Silent Brothers, keepers of knowledge who have their mouths sewn shut. Charming.
In City of Ashes, we see a new purpose for the Silent City: prison. Jace is held there by the Inquisitor, and it's not as much fun in this jail as Orange is the New Black makes prison seem.
Jace isn't there for long. Valentine brings his pet fear Demon (an evil boy's best friend) to the Silent City, kills all the Silent Brothers, and takes the Soul-Sword.
The Silent City isn't that silent when all this is happening. Consumed by fear, the Silent Brothers tear the stitches from their mouths (ouch) and scream noises worse than Mariah Carey trying to sing, "All I Want for Christmas is You" in 2014.
Although the term "City of Ashes" is never used in the book, we can only imagine that is what the Bone City will become without its guardians.