Bruce Wayne is kidnapped whilst protesting the use of Arkham City, a dilapidated part of Gotham that is used to house the resident of Arkham Asylum. He is trapped inside the facility, forced to fight the most powerful criminals in Gotham, and confront Hugo Strange.
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Nolan North voices both Penguin and Black Mask here, on top of a big chunk of the randomised inmate chatter you hear wandering around the city. So it's entirely possible to hear Penguin bark orders at his goons and then have a random thug reply in what's technically the same voice actor's other performance.
The Riddler really went all out for the sequel, hiding a total of 440 Riddler Trophies across Arkham City on top of 40 separate riddles to solve. Some of them are set aside specifically for Catwoman and glow a different colour, since Batman gets electrocuted if he tries to grab those ones himself. It roughly doubles what Arkham Asylum threw at players, which tells you a lot about Rocksteady's confidence in the bigger map.
When the game came out, rumours spread fast that players had spotted Man-Bat hiding somewhere in Arkham City. People combed through the files looking for him, but there's genuinely nothing there. Not a single line of dialogue or asset referencing the character exists in this one, it was purely players seeing what they wanted to see.
Visit Calendar Man in his cell and change your console or PC's system date to match certain real dates, and he'll come out with a unique bit of dialogue tied specifically to that day. One of his lines even tells Batman he'll 'be at your end,' which is exactly the kind of cryptic nonsense you'd expect from a villain whose entire gimmick is dates.
Black Mask had only ever been referenced through a scavenger hunt clue back in Arkham Asylum, so getting to actually see him locked up in Arkham City felt like a step up. He's not important enough to warrant his own boss fight here though, he's just one of the many prisoners you spot wandering around early on, saved for a bigger role down the line.
Looking back across the series, Zsasz's expanded role kept getting trimmed further in later games too, with unused audio files turning up online of him interacting with Batman in scenes that never made the final cut. It shows Rocksteady clearly liked the character a lot more than what actually reached players.
Climb around Rome's rooftops long enough and you'll eventually hear a passing NPC mutter 'another capering crusader' under their breath. It's a pretty blatant jab at Batman, dropped into a game about a guy who spends most of his time jumping across rooftops in a hood.
