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Diamond City is a city appearing in the WarioWare series. Wario claims that it is "his city" that he has made into a "sparkling gem of a metropolis." Here, Wario and his friends have parties, and most of them reside here.
It isn't clearly known where Diamond City is located. The WarioWare series suggests that the city is possibly located on Earth (mostly from various statements made by the alien Orbulon). A map of Earth that can be seen in Wario's House would seem to support this assumption. Earth is also seen in the opening cutscene of WarioWare Gold, significantly resembling the real world Earth as opposed to the Earth seen in main series Mario games such as Super Mario Odyssey. However, the Diamond City race course in Mario Kart Arcade GP features symbols such as E. Gadd's logo and Magic Paintbrushes, suggesting that the land may be on the same planet as that of the main series Mario games.
In WarioWare: Twisted!, it can be seen that Diamond City is located on an island.
Mario Kart series
Names in other languages