In the courtyard to the right of the starting point (the castle), there is a grate. Open it with your sickle to access the underground area. There will eventually be an underground sewage waterfall there. To the left of the ladder is a hand-cannon that allows you to fire grenades and a cracked indentation on the wall. Save your game just to be safe, then back away from the wall and fire a grenade at it. Go in the empty area you just opened to get this achievement, which will be accompanied by the in-game message "Secred found!" [sic]