To get new realms to fight at, do the following.

Realm	How to unlock
The Castle Stronghold	Get 125 Purple Crystals.
The Desert Realm	Get 200 Yellow Crystals.
The Dream World	Get 225 White Crystals.
The Forest Realm	Get 175 Green Crystals.
The Foridden Province	Get 15 Orange Crystals.
The Mountain Kingdom	Get 100 Red Crystals.
The Sky Realm	Get 150 Blue Crystals.