The Fortress of Solitude is a fictional fortress appearing in American comic books published by DC Comics, commonly in association with Superman. It is the place where Superman first learned about his true identity, heritage, and purpose on Earth. The fortress functions as a place of solace/occasional headquarters for Superman and is typically depicted as being in frozen tundra, away from civilization. Its predecessor, Superman's "Secret Citadel", first appeared in Superman #17, where it was said to be built into a mountain on the outskirts of Metropolis. By issue #58 it is referred to as the Fortress of Solitude, seems at a glance to be a freestanding castle, and is said to be located in a "polar waste". When the Fortress reappears in 1958 and for the first time takes center stage in a story, it is again an underground complex in a mountainous cliffside.

Superman of Earth-Two is an alternate version of the fictional superhero Superman, who appears in American comic books published by DC Comics. The character was introduced after DC Comics created Earth-Two, a parallel world that was retroactively established as the home of characters whose adventures had been published in the Golden Age of comic books. This allowed creators to publish Superman comic books taking place in current continuity while being able to disregard Golden Age stories, solving an incongruity, as Superman had been published as a single ongoing incarnation since inception. This version of the character first appeared in Justice League of America #73.