The Secret Citadel
Deep in the mountain range outside the city of Metropolis on Earth-2 lies Superman's secret mountain retreat known as the Secret Citadel, where the Man of Steel can relax, perform scientific experiments, and escape, if only temporarily, the demands and distractions of the everyday world.
Superman has noted that the mountain peak is located "where even a mountain goat would have a hard time reaching it!" (SF No. 217/1, Apr 1982: "Back to Square One!").
Here in this secret sanctum, far from civilization, are the fabulous trophy room, housing the hard-won memorabilia of more than a thousand adventures; the workshop and super-laboratory, where Superman labors in search of an antidote to Kryptonite and performs other experiments; the gymnasium and recreation facilities, where Superman exercises, relaxes, and indulges in a variety of super-hobbies; special rooms and memorials in honor of Superman's parents, foster parents, and closest friends; special monitors for communicating with distant planets and alien dimensions; and numerous other rooms, exhibits, weapons, machines, and scientific devices. Indeed, since the invasion of the Citadel by an outsider could result in the placing of these devices in the hands of evildoers - as well as endanger Superman's secret identity - the exact location of the Citadel remains one of the world's most closely guarded secrets (See also Fortress of Solitude).