Kandor was founded thousands of years ago when the ancient Kryptonian's advanced far enough to warrant urbanization. It consistently grew lager over countless generations, over time becoming the dominant settlement of the planet.
Thousands of years later, Jor-El traveled to Kandor for a council with the Kryptonian Law Council about Krypton's impending core collapse due to the thousands of years of strip mining, arguing that the planet must be evacuated. Just as they denied his request, General Zod blasted his way into the chamber in a coup d'état. He had Jor-El arrested while he shot High Eminence Ro-Zar. Jor-El escaped and stole the Growth Codex, retreating to his homestead Citadel outside of Kandor.
In the DC Comics universe, Kandor was shrunken and stolen by the supervillain Brainiac, several years before the planet was destroyed. Upon its recovery by Superman, it has been traditionally kept and monitored in the Fortress of Solitude.