Koridai is one of the Territories of the Koopa Empire. Geographically, it is a small island and is the smallest of the Territories (only 32 km2). However, it is all urbanized and lotsa people live here. The capitol (and only city, as the whole thing is a city) is Koridai. The Viceroy is Wendy O. Koopa, one of Bowser's 8 kids. Before it belonged to the Koopa Empire, Ganon ruled Koridai along with Gamelon (Which he also lost to the Empire). Then, Bowser's Army invaded Koridai and took it from Ganon, and banishing him to the Dungeon. It is located in the Hyrule Ocean in the Mushroom World.
Koridai is one of the oldest places in the Mushroom World. It was first settled in 600 BCE by an ancient civilisation called the Wise Ones. They carved the faces of their gods into the mountains of Koridai. The Wise Ones lived in peace and isolation for many years until 301 CE, When explorers from Hyrule came and claimed the land for themselves. Hyrule banished the Wise Ones and sent 10,000 colonists. They occupied Koridai until Ganon came and took over. He carved his own face into Koridai and joined the Wise Ones' gods. He ruled as a dictator for a while until Bowser came and conquered Koridai along with Gamelon from Ganon, who was banished to live as a hobo in the Mushroom Kingdom.
Bowser realized that he had alot of rich and important people tired of living in crowded cities full of nothing but workers. So he decided to build a huge city on Koridai that was built into the mountains to send all the rich people to because it would solve their problems and look cool. Bowser then realized that a city built into some mountains would cost too much, so he blew up 3/4 of the mountains and destroyed all but two of the Wise Ones' carvings aswell as Ganon's Face. He built a huge modern city on the new land and built the city into the mountains on the remaining mountainous section.