Capital City Prison
The Capital City Prison or Prison Tower is a large, fortified stone tower located inside a hollow volcanic crater near the capital. It is here where the Fire Nation keeps its most highly prioritized criminals. As shown in "The Headband", Iroh is one of the few prisoners being held within the penitentiary, until he managed to break himself out as seen in The Day of Black Sun, Part 2: The Eclipse. Some Kyoshi Warriors have been imprisoned there too.
After Fire Lord Ozai was defeated by Aang he is incarcerated there. Inside the prison a cell consits of a stone room then a cage in which the prisoner is held. This design allows guards to enter without being attacked (it should also be noted that even though the prisoner is locked in the cage, the actual cell door is generally left unlocked).