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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. However, the Diamond City race course in Mario Kart Arcade GP features symbols such as E. Gadd's logo and Magic Paintbrushes, indicating that it might be somewhere outside Earth.
In WarioWare: Twisted!, it can be seen that Diamond City is located on an island.
Mario Kart series
Names in other languages