Fort Rozz was the Supreme Headquarters of the Kryptonian Defence Force, and unquestionably the most fortified structure upon Krypton.
General Zod later brought Fort Rozz out of the Phantom Zone to Earth and used it as his headquarters. He later decided to upgrade it with a governing A.I. much like Sentrius. Superman would later use this in a plan to defeat General Zod by tampering with the bio-coding in the systems so that Fort Rozz wouldn't identify Superman's allies as hostiles, so when Zod caught wind of it, he was unable to command it to attack them. In the end, Superman issued a self-termination order on Fort Rozz, destroying it forever.
In DC Comics, Fort Rozz was a holding facility for Kryptonian criminals, pending their sentenced exile to the Phantom Zone. In Post-Crisis continuity, Fort Rozz was accidentally sent into the Phantom Zone during a prison riot and existed as a bizarre anomaly.