You unlock the Riddlers' hideout.
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They are the riddles and riddler trophies.
405 trophies i think
You need to find various Riddler trophies and interview tapes hidden throughout the entire game. Plus, if you find all of the trophies, interview tapes, and all of the Riddler's challenges you unlock an achievement (I'm guessing there is a trophy on PS3 that's the same). Games Radar has an amazing Riddler challenge walkthrough. http://www.gamesradar.com/f/batman-arkham-asylum-riddler-guide/a-2009082614217603032 I suggest beating the game first, however, before doing the Riddler's challenges.
well after batman arkham city is arkham origins so yes there is
Arkham Asylum came before Arkham City Asylum came out in 2009 City came out in 2011