served similar purposes. Built into the side of a steep cliff, the Fortress was accessible through a large gold-colored door with a giant keyhole, which required an enormous key to open it. The arrow-shaped key was so large that only Superman (or another Kryptonian such as Supergirl) could lift it; when not in use, the key sat on a perch outside of the Fortress, where it appeared to be an aircraft path marker. This was until a helicopter pilot followed the direction of the arrow straight to the entrance of the Fortress, forcing Superman to develop a cloak to camouflage the entrance and key (which now hung on brackets on its side beside the door) and to ensure the Fortress's secrecy. The Fortress contained an alien zoo, a giant steel diary in which Superman wrote his memoirs (using either his invulnerable finger, twin hand touch pads that record thoughts instantly, or heat vision to engrave entries into its pages), a chess-playing robot, specialized exercise equipment, a laboratory where Superman worked on various projects such as developing defenses to kryptonite, a (room-sized) computer, communications equipment, and rooms dedicated to all of his friends, including one for Clark Kent to fool visitors. As the stories continued, it was revealed that the Fortress was where Superman's robot duplicates were stored. It also contained the Phantom Zone projector, various pieces of alien technology he had acquired on visits to other worlds, and, much like the Batcave, trophies of his past adventures. Indeed, the Batcave and Batman himself made an appearance in the first Fortress story. The Fortress also became the home of the bottle city of Kandor (until it was enlarged), and an apartment in the Fortress was set aside for Supergirl. A detailed depiction of the Fortress and its contents forms the background to DC Special Series #26 (1981); "Superman and his Incredible Fortress of Solitude", in which Superman minutely inspects the Fortress, suspecting an enemy has planted an Earth-destroying bomb within it. Another noteworthy appearance of this version of the Fortress was in 1985's Superman Annual #11, a story by Alan Moore and Dave Gibbons titled "For the Man Who Has Everything", in which it served as a battleground for Superman, Batman, Robin, and Wonder Woman against the alien would-be overlord Mongul. This story was adapted to animation in the 2004 TV series Justice League Unlimited. In addition to Mongul, the Fortress