The image of the STAR Labs Hall and Hangar building that supposedly becomes the Hall of Justice was a CGI manipulation of photos of the old Union Station in Cincinnati, Ohio. The former train station (now a museum) was the inspiration for the Hall of Justice in the original 70's Super Friends (1973) cartoon. When Barry tells Thea he owns the hall and hanger, she tells him he should do something with it which might foreshadow a CW version of "Justice League".
The second of a four night crossover between the CW shows Arrow, The Flash, Supergirl and Legends Of Tomorrow. The crossover began in Supergirl episode 2x8 "Medusa," though that was more of a teaser at the end of the episode (featuring the scene of Barry and Cisco recruiting Supergirl), and continues after the Flash episode in Arrow episode 5x8, also called "Invasion!," and ends with the Legends episode 2x7 "Invasion!"
This marks the first live-action appearance of the future "Hall of Justice", which was the headquarters for the animated series Super Friends (1973).
Barry tells Kara that she's not the first superhero to be mind-controlled. In the first Flash/Supergirl crossover episode, Kara was dealing with the fallout from being mind controlled by red kryptonite, and Barry recalled when he was controlled in a similar fashion. The latter incident also occurred during a crossover story between the Flash and Arrow.
Kara Danvers' first appearance in the Arrowverse. Though Supergirl (2015) crossed over with The Flash (2014) in "Worlds Finest (2016)", that episode exclusively featured Barry on Kara's Earth, not the other way around.