Once the main statue is illuminated, there will be a puzzle to complete by clicking on the mechanical bird. Once completed, a clockwork gear will drop. Pick it up and return to the sundial scene. Zoom in on the box by the sundial and use the gear on it to open the box. Inside is a pocket watch. Take the watch back to the mechanical bird statue and place it. The main gate will become illuminated. Zoom in on the gate and complete the puzzle to open the door. After watching the cutscene, the achievement will unlock.