Batman: Arkham City Cheats

Catwoman DLC Trophies
Arkham City Sirens (Silver): Drop in on an old friend
Campaign Kitty (Bronze): Obtain all 108 medals on the original Arkham City campaigns (as Catwoman)
Family Jewels (Silver): Retrieve your stolen goods
Feline Revenge (Bronze): Obtain all 72 medals on the original Arkham City ranked maps (as Catwoman)
Pick Pocket (Silver): Steal the score of a lifetime
Sphinx' Riddle (Bronze): Complete all 40 of the Catwoman Riddler grid items

Nightwing DLC Trophies
Campaign Nightwing (Bronze): Obtain 114 medals on the original Arkham City and Nightwing Bundle Pack campaigns (as Nightwing)
Nightwing Revenge (Bronze): Obtain 78 medals on the original Arkham City and Nightwing Bundle Pack ranked maps (as Nightwing)

Robin DLC Trophies
Campaign Wonder (Bronze): Obtain 114 medals on the original Arkham City and Robin Bundle Pack campaigns (as Robin)
Robin Revenge (Bronze): Obtain 78 medals on the original Arkham City and Robin Bundle Pack ranked maps (as Robin)

New Game+
New Game+ allows you to play through the story again on a harder difficulty with all of the tools you already acquired in your first game. Note that this will not prevent you from playing your original save as well -- the New Game+ is separate.

New Game+: Beat the story mode on Normal or Hard difficulty.

Hidden Message
Using the Cryptographic Sequencer, locate coordinates 500,900. You will intercept a frequency repeating the number sequence "5-15-9-7-21-18-18-14-3-5-24-15-12-7-22-3-10-5-15-9-22-3-8-25-26-15-16-25-10-15-17-25". To decode, 1 = C, 2 = B, 3 = A, 4 = Z, and so on. Decoded, the message reads "YOU WILL PAY FOR WHAT YOU HAVE DONE TO ME".

Scarecrow Hideout
Go on top of a boat near the industrial district (the boat should be near two cranes). While on the boat, take out the cryptographic sequencer and use it. Letters will appear instead of the radio stations. Turn the letters until it reads "City Of Terror." A secret passage on the boat will open. Go down into the boat to find hundreds of roaches, a medical order form addressed to Jonathan Crane, and a Scarecrow victim who will squirm if you zoom in on him.

Cryptographic Message *SPOILERS*
Using the cryptographic Sequencer, set the coordinates to 200, 500. A voice will speak the following numbers: 3-20-26-18-26-16-24-1-11-4-24-9-3-8-5-2-12-18-6-16-7-11-3-10-17-5-13-4-21-8. To decode requires a bit of work. First set each number to a letter in the alphabet but in reverse so that Z=1, Y=2, X=3 and so on. From there you use a special cipher known as a Vigenere Cipher that requires a keyword to translate. By using the keyword "scarecrow", the message is translated to "FEAR WILL TEAR GOTHAM CITY TO SHREDS". Due to the keyword being scarecrow, it could be implied that Scarecrow is behind at least this message and possibly more.

Easy way to get the Flawless Freeflow Fighter 2.0 trophy
Go to the Riddlers Challenges and go to Custom Challenge. Select the first map and under modifiers choose Takedown Projectiles. This will make it so that your batarang will knockout enemies in 1 hit. Spam the batarang through all 4 rounds knocking out the enemies without getting hit once and you will get the trophy.

Play in Any Costume, Even Before Finishing the Game
To unlock this, simply press the following combination on the Main Menu screen (after you have selected your save slot): - Left, Left, Down, Down, Left, Left, Right, Up, Up, Down. You will get an audio confirmation and when you continue your game, you will get the option to select your favorite skin.

Taunt Animation
To make Batman (or any other playable character) execute a "taunt" like they do in the beginning of a challenge map or round, rotate the right analog stick clockwise two or three times. If you execute a taunt before performing a glide kick, the camera will change into the enemy's perspective during the glide kick action (only works for Batman or Robin).