Halloweentown is the home of all Halloween-themed creatures. To avoid doing any more harm to the humans, the monsters created their own world. However, there were rogue elements that plotted to undo the peace in some way; Kalabar sought to rule the human world; Kal sought to use the Creature and Grey spells to reverse the inhabitants of both worlds; Galloway sought to seal the portal and isolate Halloweentown; the Dominion sought to ensure the eternal rule of witches and warlocks over all other creatures.
In the Beginning
Following the founding of Halloweentown during the Middle Ages, a group of witches and warlocks called the Dominion planned to use the Cromwell Gift owned by Splendora Agatha Cromwell to make themselves rulers of the world. However, Splendora managed outwit them by locking the Gift in a box and burying it deep below Cromwell Castle; she gave the key to her future granddaughter from the 21st century, Marnie Piper. This left the Dominion without a way to conquer Halloweentown for centuries. A jack-o-lantern was left by Marnie to be put in the town square.
From then on, Halloweentown would evolve along with the human world, having all the technological and medical advances humans would in the coming years. A portal between the two worlds would open every Hallows Eve, or Halloween as it would later come to be known as. Eventualy, a bus would be used to traverse between the worlds.
Residents of this world had been vanishing without explanation after becoming hostile. The Cromwell family investigated the matter, revealing the warlock Kalabar had been behind it all as part of his plan to lead all the monsters back to the human world in a bid to conquer it. However, they used Merlin's tailsman to break his spell over those he kidnapped and to destroy him with its powerful light. During this time, Halloweentown's time was not synchronised with the human world's allowing any of the residents to visit the world so long as it was not Midnight - in other words, the start of November.
Time in Halloweentown had synchronized with the mortal world, putting a timelimit on when people could cross between the two worlds. Kal, son of Kalabara, cast the Grey Spell on the world, turning it monotone and the monsters into humans; the spell made monsters into what they views humans as, which was boring and dull. It would seem this would had a beginning as well; Marnie ended up in a pre-Halloweentown forest during a time travel attempt to escape Kal's trap. The Cromwells broke the limitation on the portal, allowing crossing between both worlds whenever they chose. With this newfound privilege, they undid Kal's magic in the human world and broke the Grey Spell in Halloweentown.
The world itself is not seen, unless the Halloweentown council's room counts.
Halloweentown returns as the main setting for the fourth movie, primarily Witch University. Due to the portal being open all the time, many warlocks and witches decided to go to college in the human world to get away from their parents like most teenagers. Because of this, the heads of the school decide to open the campus to all monsters of Halloweentown and to make things fair, forbade the use of magic.
The descendants of the original Dominion tried using Marnie to try unlocking the box Splendora had locked the Gift inside. They succeeding with the threat of leaving Dilan permanently a dog; they reversed the spell as part of the deal. However, their plan backfired as they exposed themselves and were captured by undercover agent Periwinkle.