The Capital City Prison or Prison Tower is a large, fortified stone tower located in the Fire Nation inside the hollow volcanic crater that forms the capital city, where highly prioritized criminals are kept.
After his capture at Ba Sing Se, Iroh was imprisoned here. During the eclipse, Iroh took advantage of the guards' lack of firebending to escape using his new buff body he had worked on while imprisoned. After Zuko declared his intentions of joining the Avatar, he went to free Iroh from prison, only to find a busted up cage and a semi-conscious Warden Poon.
After the failed invasion on the Day of Black Sun, the prison was used to house many of the captured invaders and war prisoners, such as Bato, until their transfer to the Boiling Rock. After Fire LordOzai was defeated by AvatarAang, he was incarcerated here.
The interior consists of various caged cells made of stone, which are watched over by guards like Ming. 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.