The Fortress of Solitude was the name used to describe the two fortresses in which Superman had his reprieves from the Earth. The original fortress was a satellite space ship that once housed the Collector of Worlds. However, this fortress ended up being destroyed, forcing Superman to relocate to the Arctic wastelands.
The current Fortress was built by unknown means but does have it's origins with that of Kryptonian technology as well as a Kryptonian A.I which guards the Fortress. The Fortress itself was responsible to for removing Superman's Kryptonian Armor when are scanning him it revealed that his DNA did not match with the record the had of Kal-El.
After Savage removed the Fortress from the arctic, he brought it to Metropolis. When Savage shot Superman after the Kryptonite infection had burned out of his system Clark fell towards the Fortress where it scanned his body opened up and returned Clark's Kryptonian Armor back to him and returning his powers to full strength.
Following the death of Superman, Clark White a Superman from a timeline deleted by the Brainiac God who was marooned on Earth 0 traveled to the Fortress with his younger self's body. He was allowed access due to him and Kal-El sharing the same DNA and consulted the Kryptonian A.I of the Fortress to bring Kal-El back to life using the Regeneration Matrix. However the device did not exist in this timeline as it did in the older Superman's.
After deciding to return to the role of Superman, Clark erected a statue of the younger Superman in the Fortress between the statues of his adoptive Earth parents.