A city planet is a subtrope of Absurdly Cool City, Mega City, and Single-Biome Planet, because you just took an entire planet and made the entire planetary biome a single city. In other words, this is what happens when someone takes Planetville a little too literally: there is only one "city" on the planet, and it covers the entire planet. Sometimes referred to as a planet city, world city (though "world city" has also been used to mean other things), completely urbanized world, omniopolis.