The Realm of the Gods is a location outside of Amaranthine itself. As suggested by its title, it is an entirely separate realm from the mortal plane. It is here that the Gods travel when they are not found in the mortal world.
It is also here that each God creates a castle in their own image. The castles are arrenged according to Elemental affinity, with Lord Nivicus' castle located in the center, and the elemental deities located at each of the four cardinal directions from the center.
However, as the Gods have banished themselves from Amaranthine proper, they have also all vanished from the Realm, leaving their many imposing castles as the only reminders of their forgotten presence.
As of this writing, the only time that the mortals of Amaranthine were permitted entry into the Realm was during the Element of Air incident.