Just as you were required to complete Wizard City before you were given Spiral door access to Krokotopia, and you were required to complete your required assignments in Krokotopia before you were given Spiral door access to Marleybone, so will you need to complete Marleybone before you are given Spiral door access to MooShu.
Wizard City, Krokotopia, Marleybone, Mooshu, Dragonspyre, Celestia and Zafaria work this way as they are main storyline worlds.
Grizzleheim and Wysteria are side worlds with their own requirements.
