The series takes place in an alternate universe from the games canon. As such there are many similarities and nods to the video games, including the fan game Sonic Robo Blast 2, but there are many notable differences.
The first notable difference is that Greenflower City was saved by Sonic from annihilation. The Black Rock has been destroyed, but it's possible that meteorite fragments have crash landed on the planet's surface...
The Greenflower Region, being the area holding all of the SRB2 content, has some expanded details and new original content. The region is seen as a very eco-friendly area with a pretty big flower motif. Between Greenflower Zone and Techno Hill, there is a large vegetation farm. Techno Hill Zone, previously known as Daffodil Hills, used to be a windmill farm that would provide power for the city, before Eggman took over and industrialized the area into a poisonous chemical plant. The windmills were torn down to make way for the large factories. The rivers were filled with slime and toxic waste, and the area became nigh unlivable for some time. Before Sonic had shut down the factory, it had actually been stealing power from the city, causing a city-wide blackout. Greenflower City wound up having to get a more traditional power plant.
Years later, Eggman makes it operational again after giving the factories power again. Sonic and Splice team up to shut it down for good, ultimately reversing the flow of the slime in the pipes to clear the rivers and lakes and flood the factories. Techno Hill slowly began to be restored to its former self.
Sonic Adventure 1
The events of Adventure 1 take place sometime between Eggman's two attempts to eradicate Greenflower City, years before Season 1. The only thing that really affects the series is the Perfect Chaos Flood. In this universe, the flood was not quite as damaging, as the city was able to mostly rebuild within a few years. However even as late as Season 3 there are still areas in the city that are still destroyed; some ruined districts while others completely inaccessible.
There were many people who took refuge in Greenflower City or Emerald Town during the flood. One of which was Tab The Cat, the sole survivor of a small family that once lived in Station Square. While some people returned to the city to try and rebuild, some stayed in their new locations to begin anew.
The Mystic Ruin and Angel Island.
The Mystic Ruins has a secret chamber near the Lost World temple containing a teleporter; it leads to a specific room in the Hidden Palace on Angel Island closed off from the rest of the area. However, the entrance to either sides of the teleporter can only be opened by those who hold the artifact that serves as the key. Knuckles holds one, but Kyle Tyris holds onto a spare. This is the only known way to travel between Angel Island and ground level without some means of flight.
Angel Island is constantly moving with the wind. It moves very slowly, but as it stays airborne it drifts off anywhere. Sometimes it's difficult to find, other times it's in plain sight. The Hidden Palace is used to keep the Master Emerald hidden. However, the island does not stay afloat while it's down there; only while the emerald is at the shrine.
The echidnas hailing from Angel Island are all but extinct, save for Knuckles who is the last of his kind. However, There are still different types of echidnas living across the globe; the most notable are the kind that originated from the Kartilla Region.
These echidnas tend to grow slightly taller, and don't normally grow long dreadlocks. Instead they have shorter, thinner, and somewhat duller quills that hang down around the sides of their heads and never grow further than shoulder length; instead their hair normally grows over the quills.
Other notable features are the common lack of spikes that grow on their knuckles. Very few of these echidnas are born with any number of spikes, ranging from 1-4 on each hand. They also tend to have white or black birthmarks on their backs instead of their torsos.
Sonic Adventure 2
There are a few references to Adventure 2's plot with the appearance and mention of some notable areas. The city from Adventure 2 was merged with Central City from Sonic Battle, so Emerald Town is indeed a neighboring town. The moon still has a hole blown through it thanks to Eggman, which has been named "Lunar Crater Zone". There is also mention of a new Prison Island after the old one was destroyed. The new one is described as being more "man-made than natural".
The most important reference to Adventure 2 is the statue of Shadow the Hedgehog in Central City, erected in honor of the hero who gave his life to save the world. As far as anyone knows, Shadow the Hedgehog is dead.
As mentioned above, Emerald Town exists just outside of Central City, and Night Babylon is somewhere in said city as well. Holy Summit is at the highest point of the Mystic Ruins, having of course some connection to the ancient echidna tribes.
Knuckles does not live there, nor is the Master Emerald kept there. There is also only one Master Emerald.
[To Be Added]
- An Idea I came up with in order to make Cymbeline's introduction (and parts of her backstory) a bit more believable. Hybrids are essentially the Half-Elves of the series. Except, for the most part, people have gone past the discrimination with laws that basically allow equality between the hybrids and pure-bloods. However, not everyone is willing to accept that. Of course in Cymbeline's case, she's a hybrid of an alien species. (Half Hedgehog, Half Angel) so she suffered quite a bit from it. Even other hybrids thought she was weird.
To be honest, hybrid characters aren't all that popular--despite inter-species romance being a big thing in the fanon. I always found that odd, but it was part of the reason why I wanted to go through with this.
The Ancient War
[To Be Added]
The Lost Civilization of Eleyrah
[To Be Added]
The Haunted Island and the Miracle Water
[To Be Added]