Beanbean Castle is a major location in Mario & Luigi: Superstar Saga. It is where Queen Bean and Prince Peasley live and rule over the Beanbean Kingdom from, and it was where the Beanstar used to rest. It also acts as the main base of operations throughout the game. It is located just beyond Beanbean Castle Town.
Beanbean Castle appears to be a large castle with many rooms much like Princess Peach's Castle.
When Mario and Luigi first enter Beanbean Castle, the town was destroyed and the front gate was blocked. However, Mario and Luigi present the guard with the Peasley Rose and the guard acknowledges them as friends of Prince Peasley and lets them in. As Mario and Luigi enter, the ambassador Lady Lima appears and wants to know who they are. Mario and Luigi tell them who they are but Lady Lima still doesn't believe them. She then knocks them down into the basement and tells them if they could fix the plumbing, then she'll believe that they are the true Mario Bros.
Mario and Luigi do so but it doesn't matter as they come across the real Lady Lima. It is revealed that the other Lady Lima was none other than Cackletta and she tricked Mario and Luigi to fixing the plumbing which actually deactivates the security on Beanstar. Mario and Luigi confront her but Cackletta soon presents them with a powered-up Queen Bean and she flies away. Mario and Luigi defeat the Queen Bean and Lady Lima reveals that she must have ingested a Belly Blech Worm and only the Chuckola Reserve could cure her. Mario and Luigi head out to find some.