This is in Chapter I, and in fact can be missed entirely if you don't follow Roche into Loredo's quarters. Go around the back when Roche tasks you with doing so and you will be stopped by a guard. Go back to Roche and speak with him about the guard. Roche will distract him, allowing you to get in. There is one guard next to a rock, so sneak over and hide behind it. Wait for him to turn around and knock him out with (if you don't knock this guy out, the other guard urinating by the door will always around and notice you). Upon knocking out the guard by the rock, go to the one urinating and knock him out too. Look to your right to see a rock that you can climb to get over the locked door. Once you drop here, make a save and just use common sense for the remainder and you'll be fine.
Once you get the kayran trap fragment (iron frame) from the chest at the end and eavesdrop on Loredo (climb the ladder and shimmy next to his window to trigger a small cutscene), climb the ladder on the roof to drop down by the campfire, where you're achievement will unlock.