Although we got our first glimpse at Superman’s “Fortress of Solitude” back in 1949, that story was still a one-off with no attempt to make it a more permanent fixture in the mythos. But “The Super-Key to Fort Superman” in Action Comics #241 (April 1958) goes really deep on the Fortress, including the key to its door, disguised as an iconic giant arrow marker in the Arctic. This time we get a pretty detailed glimpse of various rooms, side projects, and hobbies of Superman and a surprise “intruder” that turns out to be Batman playing a gag on Superman.