This is a very simple achievement. In order to get it, all you need to do is:
Go into your menu ---> Collectibles ---> DLC ---> Clockwork City
*** Spoiler - click to reveal *** This will open up a portal to Mournhold. You will then meet up with Divayth Fyr at the Tribunal Temple, after which you will follow him through the waterfall into a Clockwork City. Upon your arrival in front of the brass fortress, the achievement will pop.