As much about the Tower, Demon Realm and Divine Realm is unknown, this page should be considered as speculation until more is revealed.
In the world The Last Sovereign there appear to be four different realms of existence: The Tower, The Divine Realm, The Material Realm, and the Demon Realm. Of the four realms, Simon and his companions live in The Material Realm and have visited the Demon Realm and now have a base situated in the Tower.
The Material Realm Edit
The material realm is apparently the biggest in size of the three realms and is also host to the struggles of the mortal races.
The Demon Realm Edit
The demon realm is a name for the shallower part of the Tower. The demon realm can be considered to be "down" in direction compared to the material realm. Though once you enter the demon realm physical directions become less applicable. A physical body has less impact on how you navigate the demon realm, and you must rely on magic. Host to the category of beings called demons, it also contains mortal souls that one way or another have entered the demon realm. One needs to be a powerful mage or be a shard bearer to safely traverse the demon realm, not only as demons will attack, but you cannot rely on physical might and must use magic to combat demons.
Sabitha is apparently from the demon realm, and was captured and subjugated at some point in time by The Fucklord. Demons and mortals stronger than her and The Fucklord are also noted to inhabit the demon realm. A mortal soul trapped there is currently trying to crawl out of the demon realm, and when it does it will try to destroy reality, otherwise known as the material realm.
The Divine Realm Edit
If the demon realm can be considered to be "down" then the divine realm can be considered "up." Little is known about the divine realm, other than it presumably is host to the goddesses.
The Tower Edit
The Tower is something that could possibly connect the divine and material realms. As Mithyn was attempting to descend the Tower to go somewhere, presumably you cannot leave the divine realm by simply physically moving or teleporting and must use the Tower, as she is shown teleporting to where the Mystery Divine Being is.