Kryptonopolis was the largest city on Krypton after the capital, Kandor. After Kandor was stolen by the alien criminal Brainiac, Kryptonopolis became the new capital. The history of the city is tied to the history of the El family, the ancestors of Superman, as the original layout was designed by Superman's ancestor, Gam-El.
Kryptonopolis is also the home of Jor-El and Lara, the parents of Kal-El. When Jor-El's researches discover that Krypton is fated to explode, Jor-El constructs an escape rocket for his young son. Jor-El launches Kal-El (the future Superman) into space shortly before the destruction of Krypton. Kryptonopolis is also destroyed in the explosion.
Unlike the cities of Kandor and Argo, which are Kryptonian words, "Kryptonopolis" is an anglicisation. The suffix "opolis" comes from the Greek word "polis", meaning city, and can be seen in the names of modern Earth cities such as Annapolis, Minneapolis, Indianapolis and even Superman's home of Metropolis. How the city was spoken of in the alien language of Krypton has yet to be addressed.
In Superman #200 (October 1967), an imaginary story has Kryptonopolis, not Kandor, shrunk by Brainiac. This Brainiac is a sympathetic scientist who takes the shrunken city of Kryptonopolis to Earth after the destruction of Krypton. On Earth, it is Kal-El's younger brother Knor-El who becomes Superman; Kal-El instead becomes Hyperman, hero of Canada.
In the 1978 film Superman: The Movie, the city of Kryptonopolis is shown in the first part of the film. While the name of the city is not mentioned in the film, it is named Kryptonopolis in the script. In this appearance, Kryptonopolis is portrayed as being constructed out of a white, crystalline material. As in the comic books, Kryptonopolis is destroyed when the planet Krypton explodes.
In the 2007 novel The Last Days of Krypton by Kevin J. Anderson, Kryptonopolis is established on top of the ancient ruins of Xan City built by the Tyrant Jax-Ur centuries before. After the capital city of Kandor is shrunk and taken away by the Brain Interactive Construct, Commissioner Zod takes his followers to the old ruins and announces that he would reestablish the capital city there. While reconstruction of the city is in progress, Zod announces that "Xan City is a mark of the past, a reminder of lost glory". He then proceeds to rechristen the city as Kryptonopolis to "stand for our whole planet, our entire people". When Krypton is destroyed, Kryptonopolis is destroyed along with the rest of the planet, while scientists are trying to make a last minute effort (ineffectual) to create arkships that would allow people to flee from the impending disaster.