Once you locate the sewer grate in Rogueport Plaza, jump on it and use Goombella's Tattle ability to learn about it. You'll then be able to enter. The sewers are a labyrinth of pipes and hidden passages. Follow the path, defeating the various Goombas and Spinias you encounter. You'll eventually reach the Thousand-Year Door itself. Here, you'll learn about the Crystal Stars and the legend surrounding them. After a brief cutscene, you'll be tasked with finding the first Crystal Star. This involves navigating back through the sewers and exiting to the east side of Rogueport to reach Hooktail Castle.
