The Capital City Prison, also known as Prison Tower, is a large, fortified stone tower located in the Fire Nation inside the hollow volcanic crater that also contains the capital city, where highly prioritized criminals are kept.
After his capture at Ba Sing Se, Iroh was imprisoned here. Prior to the eclipse, he secretly began a rigorous workout, able to regain a significant amount of strength. Using the eclipse and Warden Poon's lack of firebending to his advantage, Iroh easily overwhelmed him and escaped. 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 Hakoda, until their transfer to the Boiling Rock. After Fire LordOzai was defeated by Avatar Aang, 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.