The Monster Kingdom is a separate realm that coexists with the reguler Earth-Realm. It encompasses all races that do not normally live within the Human Realm, or live without human knowledge of their true identity.
The Royal Family rules over the Monster Kingdom, and holds the ultimate say in all matters. There are also smaller governments that rule over their respective race, such as the Werewolves and the Vampires. Royalty has decreed that Monster Kingdom stay secret from the Human Realm, probably due to how humans viewed and acted towards other supernatural races in the past.
All Royal courts and battlegrounds are located within the Monster Realm.
Vampires are known to have the ability to quickly travel between the two Realms, and are also known to dwell in large vampire cities within the realm.