The inmate rooms inside of Arkham Asylum contained nods to several Batman villains on each door except for one that references writer and former showrunner Marc Guggenheim.
Oswald Cobblepot, Pamela Isley, Basil Karlo and Edward Nigma are the real names of Penguin, Poison Ivy, Clayface and the Riddler respectively.
Some of the locations in Chicago that were filmed in this episode were the same locations used in The Dark Knight trilogy to replicate Gotham City.
Bruce Wayne, or Batman, is constantly referred to during the episode. He has been referenced by numerous people before, including Rip Hunter, Kara Danvers, Winn Schott etc.
The wifi password Kate gave to Oliver, Barry and Kara references Batman's butler Alfred Pennyworth.
Nora takes a freeze ray from glass case that's labelled Fries, V. This is a reference to her husband Victor Fries, or Mr. Freeze, one of Batman's greatest villains.
When Oliver (as the Flash) misaims a lighting bolt, the gas released falls out of a box labelled "Crane, J". This is a reference to Jonathan Crane, a.k.a. the Scarecrow, one of Batman's greatest villains.
Digger Harkness' boomerang, Bane's mask from The Dark Knight Rises, Hugo Strange's sunglasses, the Riddler's cane, the Penguin's umbrella, and other items can be seen inside of Deegan's lab.
Kara finds a William Shakespeare bust in a box which is a reference to the 1960's Batman TV series where it has a button to the entrance of the Batcave.
A Gotham police officer mentions Nolan and Burton streets. This may be an homage to Christopher Nolan and Tim Burton, two directors of Batman films.
The Earth-Ninety Barry mistakes John Diggle for John Stewart who in the comics is one of the characters who become Green Lantern.
Barry narrates the opening introduction (using the name Oliver Queen) due to the altered reality where he's Oliver Queen/Green Arrow.
Stephen Amell and Cassandra Jean Amell are husband and wife in real-life.
At the end of the episode, the Monitor reveals that he is preparing them for a "Crisis". This ties in with the red skies emerging in the first two parts of the crossover. Both are a reference to the Crisis story arc, which includes Crisis on Infinite Earths, Infinite Crises, and Final Crisis.