From Sonic Retro
Planet Freedom is the world Sonic OVA takes place in, separated into two "dimensions". The Japanese version refers to this as two different regions, which is probably the most correct term to use. This article will use the term "region". The first region of the planet is the Land of Darkness and the second is referred to as the Land of the Sky. There are several cities and other "Zones" found within these regions.
The Land of the Sky
This is the region where Sonic and his friends, and the President and his colleagues live. It seems to be made up of sky-bound islands, making aircraft a necessity to move from place to place. All of the islands are connected to a glacier, which anchors them to the planet's surface below.
The region that is ruled by the President of South Island. It is made up of many islands including the President's unnamed city and Sonic's unnamed island.
This large city is the location of the Presidential House. The city is built on a series of sky islands that are surrounded by clouds. There are some interconnecting bridges that connect the islands and some of these islands are developed and have skyscrapers and other modern structures on top of them.
Sonic's Island is the island where Sonic is seen in the beginning of the film. Similarities to the Sonic games are apparent due to the familiar looking flowers and the Crabmeat badniks crawling about.
Land of Darkness
This is the region that Robotnik lives in. This is the actual surface of Planet Freedom and probably earned its name due to the sunlight being blocked out by heavy cloud cover and the continents that make up the Land of the Sky. Despite being called the Land of Darkness, there is plenty of green plant-life, though the land is a bit rugged with ridges and mountains. There are two known entrances here: a whirlwind like portal surrounded by clouds and a warp zone.
A possibly abandoned major metropolitan city that is dilapidated with cracked and damaged roads and buildings, some with moss covering them (one in particular looking eerily similar to the Empire State Building, implying these may be the ruins of New York City). For some reason, there is electricity as the stoplights seem to function. It is located near Robotropolis. Due to the Robot Generator being out of control and causing reactions underground, part of the city is half submerged in the ocean.
Named Robotropolis in the English dub, this is the city where Eggman lives and reigns. Its power supply is handled by a Robot Generator, which is a part of a major plot point in the film. Eggmanland is an amazing sight to see at nights, as it glows due to all the street lights. The structure in the middle has a string of lights which form a caricature of Eggman's face.