Fortress of Solitute, Secret Citadel, Etc Etc

A discussion on superhero headquarters would not be complete without a consideration of the Mythical Mansions of the Men of Steel.

The silver age Superman Kal-El had bases on an undersea cliff of Earth-One as well as an asteroid in space before finally settling on his Fortress of Solitude in the Arctic. There within the Fortress, he designed a section for his cousin Supergirl. Accessible by a gigantic golden key, it housed various chambers with memorabilia of Krypton and its owners’ exploits as well those of their family.

The golden age Superman Kal-L initially had his residence in mountains outside Metropolis of Earth-Two, known as the Secret Citadel. While he had other locales such as an Arctic palace in the late 1940's and another secluded mountain base for his cousin Power Girl with a large key to open it like Kal-El’s, which like Supergirl’s one-time desert fortress she quickly abandoned.

And, lest we forget, Krypto the Superdog had his own Doghouse of Steel... most likely converted from Kal-El's original asteroid headquarters.

Of interest is the fact that deep under the surface of Earth-Two was the Ultra-Humanite's subterranean headquarters, much like the under-the-sea Fortress of Earth-One, from which Ultra accessed his Superman's Secret Citadel. 

A connecting subterranean Sinister Citadel in Washington State, connected by a thousands of miles corridor from underneath the Secret Citadel on Earth-Two, correlated with Brainwave's hidden laboratory within a mountain outside of Earth-One's Metropolis! This lab was used as an access point to Earth-Two's Sinister Citadel, home of Brainwave's Secret Society of Supervillain, within the Himalaya mountain range in Asia. This Citadel was short lived, as the golden age Superman and his Justice Society allies, along with the visiting Justice League, evicted its supervillain inhabitants.