(This is for the fanfiction "Crossover Together". Info may change from canon information)
Unlike most other realms, the Overworld can easily be accessed directly without the aid of magical spells, as one can reach an area of the Overworld through the ability to fly.
The Overworld is mainly made up of sky and clouds, and as such land is very few and far between. Most permanent residence either remain within one settlement their whole lives or have to fly from one landmass to another. Settlements are held afloat by self-sufficient magical energy from the area, and the more energy present, the more elaborate a settlement can be. Places such as Elysium have very high amount of magical energy, whereas settlements like Synapse hold a smaller amount.