Probably the most well-known entry on this list, Rocksteady proved their prowess at world-building back in 2009, when a full year after release fans realised you could blow away one of the walls and access the plans for Arkham City, proving the developer were planning their first followup even before Asylum had become critically acclaimed. To access, head back to the Arkham Mansion (it's where you first take down Zsasz), then head past there and into the room with the huge Sharp painting above the fire. From here, facing the painting, look to the wall in the corner (seen best in the attached video), where although it doesn't look like much, a well-placed amount of explosive gel will blow straight through and allow you to walk in. https://www.youtube.com/watch?v=LGKEJIQF044 Rocksteady would continue this trend into their following games, albeit in slightly less secretive ways. Arkham City's Industrial District has a barge containing multiple items belonging to Scarecrow (one of the villains in Arkham Knight), and 2015's release features models of Arkham Asylum and City themselves - the latter visible in detail if you venture above the city skyline and look out towards the rest of Gotham.
