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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.
What else can I help you with?
How long is Batman arkham asylum?
About 6 - 8 hours of gameplay.It has replay value though because there are tons of stuff to unlock through out the game.
How do you unlock bonus costumes in batman arkham asylum?
The only alternate suit I have seen is the Armored version, unlockable after beating the games story mode. Sadly, it is only for Challenge mode.
How do you get to Mr freeze is in batman arkham city?
you dont unlock him but you do help him in the middle of the game
What happens if you get all riddler trohpies in batman arkham city?
You unlock the Riddlers' hideout.
How do you unlock the skins in Batman Arkham city ps3?
You'll have to buy the costume pack in the ps3 store.
