The outside of the Poshley Sanctum looks similar to a museum. It has four large columns and a giant golden door.
The inside of the Sanctum is incredibly massive with numerous of flags around the top. There is also a giant painting that can be accessible through a Warp Pipe. The second area of the Sanctum is brighter and contains many Dark Boos that lurk around.
Mario and his friends head toward the Poshley Sanctum to get to the Crystal Star but it is locked and the owner will be back soon. Suddenly, Pennington arrives and reveals that he is the owner of the Sanctum and unlocks it. However, the Shadow Sirens and Doopliss have already got to the Crystal Star first and left. Fortunately, that Crystal Star was a fake and the real one is hidden somewhere in the Sanctum. Mario, using his new Spring Jump ability, eventually finds the Warp Pipe leading to the painting and collects the Crystal Star.
Bowser and Kammy Koopa also enter the Sanctum in attempt to get the Crystal Star. However, like the Shadow Sirens, they were too fooled by the fake one and Pennington even said that "Luigi" had taken it much to Bowser's displeasure.