From the Super Mario Wiki
Flipside Tower is a white, monumental structure located in the center of Flipside in Super Paper Mario. It is also the highest accessible floor in Flipside. It can be accessed via an elevator located on the second floor, or by using the Return Pipe. If the player jumps off of the tower, then they will land on the third floor.
The Flipside Tower acts as a central hub between different dimensions. Each dimension is accessed via a Pure Heart doorway. A Pure Heart doorway will not be present unless the corresponding Pure Heart is placed inside of an empty heart pillar. The Flipside Tower has access to seven different dimensions.
The Flipside Tower is the first part of the town that Mario has access to, and is a pivotal part of the plot of Super Paper Mario, being the first playable area in the game, as well as the place where Mario meets Merlon for the first time. If the player decides to load their save file after beating the game, they will start on the Flipside Tower. The Flopside equivalent of the Flipside Tower is the Flopside Tower.
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