In City of Boneswe 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 Blackmakes 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.