After a long debate (a very long debate with a lot of magic being thrown around), the Villains have decided to have their takedown headquarters at Maleficent's castle.
The castle will be placed within the free green space behind the current Fantasyland, shown in the top picture. This is the current fallout zone for the Magic Kingdom fireworks (shown in the picture below with the yellow square), so the fallout zone will be pushed further north so the castle does not lie within the fallout zone. Just in case, the architects will make sure to make the castle flame-resistant The entrance will be by Gaston's Tavern and will be a stony walkway that will resemble Maleficent's castle and that goes upward (and northwestern in direction towards Haunted Mansion). The castle will be surrounded by large rocks similar to the one in the picture to depict a gloomy scene. In the distance, when you are walking on Main Street, guests will be able to see the castle appear in the distance. This portrays the impending doom of Disney's Villains and how they are also a part of Disney.