Batman Arkham City

*Minor Spoiler*

Story related, you get this during your first playable portion of the game as Batman. During this section you have to save a police officer, find the secret hideout entrance, interrogate 3 thugs and then you will open the door. When you use the Cryptographic Sequencer on the panel it will unlock.

To earn this achievement, you must locale three thungs within the Steel Mill and interrogate them.

Here are the locations:

-The first thug, which I considered to be the hardest, is close to three snipers near where you first entered the steel mill at the beginning of the flashback for Batman. You need to take out all three snipers first. My advice is to either sneak up behind them and silently take them down or use your disruptor and take out their use of the gun before taking them out, which is what I did, just to be on the safe side. Once all three are out, you may go ahead and freely interrogate him, without any harm done to you, and he'll reveal his code.

-The second thug is located on an isolated area with thugs that you'll have to do battle with, as in hand-to-hand combat. Make sure you eliminate all other thugs first before interrogating him.

-The third and final thug is surrounded by his own, but with guns. In fact, all are armed with guns. What I did was wait at a distance and used my disruptor to disable all of their guns (took awhile to wait because of the charging process for my disruptor), then I easily bested them in combat. Make sure you knock them down quick, then use a ground pound. Take the others down, rinse, wash, repeat until the final guy gets to be interrogated.

It's like dealing with the Riddler thugs during the original campaign, so it shouldn't be too bad. This is my recommended strategy. Once you earn all three codes, head back to the area where you fought your first Harley Quinn informant and his gang, use the Cryptographic Sequencer to unlock the code. The door opens and your achievement is unlocked.

Have fun and good luck!