From the Super Mario Wiki
Beanbean Castle is the home of Queen Bean, Lady Lima, Prince Peasley and possibly other royalty and servants. It is the center of the Beanbean Kingdom. Once the Bros. get to the castle they find two Beanbean Knights who try to play along with the Mario Bros., yet have absolutely no clue what they're talking about. Later, to earn Lady Lima's trust and get to Queen Bean, the Bros. must fix the plumbing under the castle. But once they fix the plumbing, Cackletta and Fawful gain access to the Beanstar. Beanbean Castle Town is the town that is near the castle and has many useful shops. The castle is key in the game, and the home of the Beanstar. The Mario Bros. come here several times in between tasks to talk to Queen Bean and the others to gain information.
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