A likely homage to Doc Savage pulps of the 1930s, Superman's Fortress of Solitude was first introduced in the '50s and has been a staple of the character's mythology since, appearing in the comics, on film, in animation and more.
And in today's issue of Superman: Lois and Clark, the pre-Flashpoint Superman revealed (to the readers) that he still possesses a Fortress of Solitude -- one secret even to this world's Superman -- and that, yes, it's still filled with goodies.
The earliest iteration of the Fortress was actually called the "Secret Citadel" and appeared in Superman #17 in 1942, according to the DC Wiki. Carved into a mountain outside of Metropolis, the Citadel made a few appearances before the first Fortress, which was located in the "polar wastes," took over. These locations were both later understood to have been on Earth-2.
The first Earth-1 Fortress, then, appeared in 1958's Action Comics #241. There, it was a massive structure carved into the inside of a mountain and located in a remote mountain range in the Arctic. Typically, the Fortress of Solitude is located in the Arctic, though more over the years, the comics have placed the Fortress in other locations, including the Antarctic, the Andes, and the Amazon rainforest.
The Fortress is nearly always "guarded" by a statue of Jor-El and Lara holding aloft a globe of Krypton, which appears in the new Lois and Clark version as well. In almost every other respect, different Fortresses and different eras have differed wildly.
Security is always a concern: in the Silver Age, the Fortress could only be opened using a key so massive only Superman could lift it. In All-Star Superman, there was a similar idea -- but instead of being physically large, it was made of a super-dense material that made a standard-looking key impossibly heavy. The post-Flashpoint Superman is currently experiencing problems related to the fact that his own Fortress's automated security attacked him and took his costume because apparently after having expelled most of his powers using a super flare, he appeared "too human" for the Kryptonian bio-sensors. the pre-Flashpoint Superman avoids that in today's issue by using a dual-biometric entry scanner (fingerprints and iris), as well as a keypad.
In the post-Crisis on Infinite Earths era from which that Superman originated, there were a number of Fortresses. In the first of them, the Superman of Lois and Clark had some experience with the technology turning on him.
That Fortress, built in the Antarctic, was a product of The Eradicator, an advanced piece of Kryptonian weaponry that was programmed to preserve the sanctity of Kryptonian life. On more than one occasion it tried to take over Superman and/or the world in order to make Earth more like Krypton. Evenutally it gained sentience, and that programming led to Superman's resurrection after he died at the hands of Doomsday.
Following Superman's return from the dead, Hank Henshaw -- on that world, the Cyborg Superman -- nearly killed the now-humanoid Eradicator and in order to save both itself and Superman, it used all of the resources the Fortress had remaining, and the Fortress itself collapsed.
That version of the Fortress contained small robot servants -- not to be confused with the "Superman robots" that would be seen in both past and future versions of he Fortress -- and a giant battlesuit from Krypton's Third Age. The battlesuit would prove key in getting the powered-down Superman home following his resurrection, and periodically popped up throughout the post-Crisis era. There is a version of the battlesuit in the Lois and Clark Fortress as well.
Another Fortress was built by John Henry Irons, using tesseract technology to create a mobile, compact Fortress, though this would be destroyed as well. A third Fortress would be set up in the Amazon, abandoned quickly and turned into a tourist attraction. The final pre-Flashpoint Fortress was set up after Superman launched a Kryptonian Sunstone into the Antarctic, the Sunstone's power creating his new base of operations that lasted until the universe was rebooted.
The post-Flashpoint Superman's first Fortress of Solitude was the ship which Brainiac traveled in, which had been reprogrammed by Superman as his new base of operations. Five years later it is revealed that the ship was destroyed during an unspecified battle and his current Fortress is located in the Arctic. How he constructed this Fortress is currently unknown -- which can also be said of the pre-Flashpoint Superman's Fortress, since all we really know is that he made it to contain Blanque.
That Fortress, it should be said, is inside a remote mountain range. Back to basics, with a wink and a nod to the similarities between Lois and Clark Superman and the Earth-2 Superman of the pre-Crisis era.
In the interim, though, it seems as though the pre-Flashpoint Superman's Fortress has become something like the Pipeline from The CW's The Flash, with villains squirreled away there. So what happens if something were to break them all out...?