Fort Rozz served as an ancient military installation on Earth-One and as a prison on the New Earth.
Fort Rozz was an ancient Kryptonian military installation. In the Kryptonian year 9846, it became a pivotal location for the Kandor-Kryptonopolis Federation during the Last War. General-in-Chief Pir-El served at Fort Rozz on the day that the continents of Erkol and Urrika launched a robattack against Kandor. General El defended his post by launching disintegrator spheres at the advancing combatants. Although his strategy was sound, the enemy retaliated by using red sun weaponry to destroy Kandor's sister city, Kryptonopolis.
In New Earth continuity, Fort Rozz was actually a Kryptonian prison. In later years, it became a temporary holding facility, containing criminals on trial before they would be sentenced to the Phantom Zone. A Kryptonian criminal known as Dev-Em was sentenced to Fort Rozz for crimes of murder and perversion, but as things turned out, a riot broke out in the prison before the administrators could activate the Phantom Zone Projector. In the midst of the chaos, the projector exploded and the entire prison was transferred directly into the Phantom Zone.
In the Phantom Zone, Fort Rozz existed as a bizarre anomaly. Unlike other elements inside the Zone, Fort Rozz maintained its mass and density and did not become an insubstantial structure like everything else in the Zone. Zoners who entered the Fort found that they could become solid, corporeal beings once again. In addition, time passed normally inside of Fort Rozz, whereas the rest of the Zone was a null-field outside of space and time. It was because of these properties that Phantom Zone criminals Zod and Ursa were able to kill the existing guards and take the fort as their own. It also allowed them to conceive and raise a son, Lor-Zod. A side effect of Lor-Zod's conception inside the fort was that he always maintained his physical form, even outside the fort's perimeters.
General Zod was able to salvage technology from inside the fort and use it to send Lor-Zod to Earth. From there, the child was able to pave the way for Zod and Ursa's escape from the Phantom Zone.
Soon after, Zod and several other Phantom Zone criminals fought against Superman and exiled him into the Phantom Zone. There, he reunited with his childhood friend, the Daxamite Mon-El inside of Fort Rozz. He also found himself in conflict with criminal Dev-Em who elected to remain in the prison while Zod and the others journeyed to Earth. Superman and Mon-El managed to defeat Dev-Em and Mon-El helped put together a rocket that could help Superman escape from the zone.
Following the Zoners failed bid to destroy Metropolis, Zod and the others (including his son) were banished once again into the Phantom Zone. Lor-Zod fled from his parents and hid inside the smaller ducts and catacombs of Fort Rozz. It was at this time that he began having inexplicable growth spurts, rapidly aging to that of a teenager.
He discovered some reverse-engineered Brainiac technology that had been left behind from when the fort was still on Krypton and realized that his parents were using it to plan their next escape. Lor-Zod donned a headband from one of the probes and this created a psionic link with Thara Ak-Var. Shortly thereafter, Thara Ak-Var appeared in the Phantom Zone and united with Lor. Using a miniaturized Phantom Zone Projector, they were able to escape the zone together.
Shortly thereafter, the Phantom Zone collapsed upon itself and Fort Rozz was (presumably) destroyed.