INTRO: Ser Wolfy (from Steam) has an amazing guide and walkthrough for all the achievements for the game. The pictures in this guide are from his walkthrough, all credit goes to him (link below).
ps. I personally followed his guide to complete the game:)
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SUMMARY: This achievement requires you to pick up a hidden key in the 2nd Library and then use that key to open a door in the Old Streets level and interact with a radio. You'll then get the achievement.
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LIBRARY LEVEL: When you get to the second Library you will do a little opening animation where you're placing a book on a bookshelf. The animation will end with you looking at some stairs, you will want to head under those stairs into the little garden area and there will be some plant boxes. One of boxes will have the key.
Picture of the stairs at the end of the animation:
https://steamuserimages-a.akamaihd.net/ugc/18470412715072472...Picture of the plant box with the key:
https://steamuserimages-a.akamaihd.net/ugc/18470412715072475...Picture of the key:
https://steamuserimages-a.akamaihd.net/ugc/18470412715072478...--------------------------------------------------------------------------------------------
OLD STREETS LEVEL: After picking up the key make your way of the library level. The next level will be the Old Streets and this will be where you do the other part of this achievement. You will want to make your way through the level until you get to a long street leading to the town hall. There will be quite a few enemies so do what you must to survive.
TIP: There is a typewriter (save point) at the top of the stairs before you go down onto the street. I also utilized the rifle (sniper) to take out enemies from afar. The SMG enemies are the main threat so sniping them really helps. Then just use your shotgun to kill the bomb robots. I'm sure you'll figure it out;)
On the right side of the street is some stairs leading to a doorway. You'll want to use the key you found in the library level to open it. It won't really give you a prompt but just open your inventory (TAB on keyboard, not sure on controller) and use the key. After it unlocks open it and go inside. You'll hear talking on a radio sitting on a bench. Interact with it and you should get the achievement.
Picture of the street and doorway:
https://steamuserimages-a.akamaihd.net/ugc/18470412715073349...Picture of using the key to open door:
https://steamuserimages-a.akamaihd.net/ugc/18470412715073352...Picture of the radio:
https://steamuserimages-a.akamaihd.net/ugc/18470412715073355...--------------------------------------------------------------------------------------------
OUTRO: Hope this guide helped, and if you have any issues I would highly recommend checking out Ser Wolfy's guide on Steam (link below).
Ser Wolfy's guide:
https://steamcommunity.com/sharedfiles/filedetails/?id=27419...