From the Super Mario Wiki
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The Sun Tower is the home of the Sun in Flower Fields. During the events of Paper Mario, Mario and his partners must travel to the end of the northwestern path of Flower Fields, past the Bubble Plant and several brier patches. At the end of the path is a large mud brick tower with a spiral staircase leading to the top. The entire area looks dried up, as the only thing growing in the area are the red thorn vines and the ground is cracked.
To reach The Sun at the top, use Bombette to destroy a rock, which is under a brick slab covering the entrance. On the side of the tower is a little doorway, in it is the rock. When the slab falls it will seal off the passage. Ascending this tower and speaking to The Sun is required before you can battle Huff N. Puff.