During Supergirl's season 6 midseason finale, "Fear Knot," with the touch of a button and a bit of Martian technological ingenuity, J'onn J'onzz turned Supergirl and friends' new Tower into the most powerful Arrowverse superhero headquarters. Surprising even his teammates, J'onn turned the facility above his street-level detective agency into an intergalactic spaceship capable of traveling into the Phantom Zone to rescue Supergirl.
The lair or headquarters is a staple of Arrowverse superheroes. Each is reflective of the hero it represents. Oliver Queen and his Arrow team had the bunker. The Legends of Tomorrow have the Waverider. Batwoman has just begun utilizing the Batcave, and Team Flash operates out of Star Labs weekly. But none of these have the power or capabilities of Supergirl's Tower.
For Supergirl, the Tower represents a group of people who have been able to mature together and shake off their pasts. Supergirl’s friends once met in an abandoned office at Catco, outfitted with spare computer parts by Winn Schott. At times they have even used Kara's loft. For a while, Supergirl operated out of the DEO’s facilities. Following the local DEO building’s destruction by Rama Khan, Supergirl's support began migrating to the simple loft above J'onn's detective agency. As the team grew, J'onn addressed their needs using his Martian technology to give the space ingenious upgrades while maintaining a level of home-like comfort. His latest addition elevates the space into a headquarters befitting the Arrowverse's most powerful team.
The headquarters of Oliver's Queen's Green Arrow, Barry's Flash, the Legends' Waverider, even the Batwoman's Batcave are in many ways a reflection of how little things change on those shows. Oliver's Team Arrow began and ended their story in the same style bunker that despite having multiple locations still remained underground and in the shadows, a place Oliver only fully left behind when he finally escaped the Green Arrow identity through death. Team Flash's Star Labs, which once housed a prison, has barely changed over the years, echoing how Barry Allen himself has failed to significantly evolve. The Batcave represents dark buried secrets and the isolation of a hero who could only find some measure of solace by living among his fears, yet the symbol of his fear ultimately overwhelmed him and his cousin Kate Kane. The Legend's Waverider, a positive example of a headquarters that doesn’t change, remains a haven where a rotating crew of heroic misfits can truly thrive.
Supergirl's Tower reflects how Supergirl and her team continue to transform as people and superheroes. J'onn’s additions to the Tower highlight the capabilities and overall potential of Supergirl and her friends. As J'onn saw Supergirl's team's needs grow he added to the Tower, even establishing a wall of heroes with a row of call buttons of their symbolic icons. To be one there is a sign of a true superhero, which was why J'onn giving Alex the name Sentinel and adding her symbol to that wall was so significant earlier this season. The Tower now contains multiple floors that include a state-of-the-art med-bay, a fully stocked armory of alien weapons, a holographic training floor, containment cells, a fully equipped lab for invention and experimentation, and a space-agency-level collection of communication equipment. And of course, it can be converted into a spaceship at the touch of a button, with J’onn using the same Martian cloaking technology he used to disguise his original spaceship as a classic convertible.
By introducing a base like the Tower, Supergirl sets up its characters for an open-ended conclusion to their stories. Having such flexibility with their headquarters means they can travel anywhere to carry out their heroic responsibilities. Their base establishes that their stories are just beginning, while Oliver and Barry's set up their stories to have more closed, more permanent endings. Making the tower a true reflection of the heroes that work and reside there makes Supergirl's Tower truly the most powerful headquarters in the Arrowverse.