The Watchtower which floats above Earth, resident base of operations for the Joker.
Justice League of America #77 (by Dennis O'Neil and Dick Dillin) features the cast of Superman, Green Lantern, the Flash, Batman, the Atom, Green Arrow, and Black Canary. The issue begins with the Justice League’s mascot at the time, Snapper Carr, being beaten up by a mob. Their reason for assaulting Carr is supposedly because he hangs out with the strange freaks of the Justice League. Carr is quickly saved by a man named John Dough, and events spiral out of control as Dough begins to manipulate Carr into an anti-Justice League crusade.
John Dough ropes Carr into a plan which involves kidnapping Batman and allowing the Atom to escape in order to later fool the Justice League. Dough’s extreme manipulation of someone who should have been a close ally of the heroes is uncanny, and Dough eventually impersonates Batman and tricks the Justice League into attending a rally where Carr testifies against them. Carr denounces the Justice League as strange outsiders and claims that people should only trust in and believe those who are truly average. While John Dough is eventually revealed to be the Joker in disguise, the team are shaken by how effectively he was able to weaponize the populace against them. Fearing the same tactics could be used to attack their Earthly base, in Justice League of America #78 (O'Neil and Dillin), the League constructs their orbital Watchtower in order to distance themselves from humanity.
While Joker is often seen as a Gotham City villain, that's only because he truly loves sparring with Batman. When his attentions turn to the Justice League, times the Watchtower itself has been attacked or compromised.
Even before he was a true DC icon, the Joker was already wreaking such ridiculous levels of mayhem that he was single-handedly responsible for the building of the Watchtower. The Justice League may act tough, but this quick and decisive move to space proves that while they may each have their own villains, it's Joker they fear above all else.
