The World Tree, which also happens to be Wizard City’s Spiral Chamber, is located inside Bartleby. The Spiral Chamber is where all of the other worlds that make up the Spiral are connected. Wizards in Wizard City can access the Spiral Chamber via Bartleby’s mouth, which leads to what looks to be a forest. A Spiral Door that allows passage between the various worlds of the Spiral is located in the middle of The World Tree.
Location: Wizard City > Ravenwood > World Tree