According to character designer Albert Feliu, a single character model in Arkham Knight could contain the same amount of polygons used to render the entirety of Batman: Arkham Asylum (2009)'s environment.
Gotham City was redesigned through building on the previous games' Gothic architecture, while making a more believable and dense city through adding minor elements like neon lights, billboard advertising, American-styled cars, and shops. Art director David Hego described the final design as: "feeling a little out of time. You couldn't pinpoint whether it's twenty years ago, now or in ten years' time."
Batman's armor was redesigned to match that of the Batmobile, using the same textures and shapes, to make them appear visually and functionally similar.
As he never said a word in a DLC in Batman: Arkham City (2011), Nightwing speaks for the first time in this video game series.
In a story continuity sense, the series goes from Batman: Arkham Origins (2013); to Batman: Assault on Arkham (2014); to Batman: Arkham Asylum (2009); to Batman: Arkham City (2011), and finally, Batman: Arkham Knight (2015). In Rocksteady continuity, the Arkham Series is a trilogy going from Asylum, to City, to Arkham Knight.