Bowser's Snow Fort is a chilly, icy fortress at the end of the Evergreen Forest. You'll encounter ice-themed enemies and slippery surfaces. Be careful with your movement to avoid falling into pits. There are several puzzles involving ice blocks that need to be melted or pushed. The Hair Dryer Thing Sticker (found in 3-11) is essential for melting large ice blocks and revealing paths.
Boss Battle: Mizzter Blizzard
Mizzter Blizzard is a giant snowman boss. He has high HP and can freeze Mario, preventing him from using stickers. To defeat him, you'll need to melt him down. The Hair Dryer Thing Sticker is incredibly effective here, dealing massive damage and removing his icy defenses. If you don't have it, Fire Flower stickers are your next best option. Use powerful Hammer stickers when he's not frozen. Be prepared for a long fight, and keep healing. Once he's melted, you'll claim the third Royal Sticker!