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Treasure Square is the hub area in Wario World. The main area, the Courtyard, consists of a circular plaza lined with patches of grass similar to those found in Greenhorn Forest as well as small gazebos containing books, which Wario may attack to save the game. The five main worlds branch off of the Courtyard: Excitement Central, Spooktastic World, Thrillsville, Sparkle Land, and the giant treasure chest where the Black Jewel sleeps until Wario has completed the other four worlds. In the section leading up to the Black Jewel's chest, there is a section of four smaller chests that allows the player to play microgames from WarioWare, Inc.: Mega Microgame$! on their Game Boy Advance (with the use of a Game Boy Adapter). These microgames are only unlocked after Wario collects all the treasures in each of the eight stages.
Treasure Square appears to be the remains of Wario's Castle after the Black Jewel destroys it in the opening cutscene and turns his treasure into monsters. Wario is seen landing here at the end of the cutscene as well as whenever the game is started subsequently. When Wario releases the Black Jewel from its treasure chest before the final battle, the Black Jewel causes the entire Courtyard as well as the branching areas to sink beneath the clouds, leaving only a translucent, circular platform to stand on. After the Black Jewel is defeated, what is left of Treasure Square disappears, and the Spritelings appear to Wario and offer to rebuild his castle.
If Wario falls into a bottomless pit or pool of water in Treasure Square or any of its sub-areas, he simply lands back in the Courtyard, instead of taking damage or falling into Unithorn's Lair as he does when inside a stage.