From the Super Mario Wiki, the Mario encyclopedia
Observation Deck is a location in the Wooded Kingdom in Super Mario Odyssey. It is located high above the kingdom and can only be reached if Mario steps on a P Switch below it and climbs up the moving platforms. A Checkpoint Flag and four regional coins can be found here, along with a seed pot which leads to a secret area in the clouds if a seed is planted there. Princess Peach, Tiara, and Glydon also appear here, but only after the game is beaten.
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