The portal opened by Secret Silver City of the Fairyland Kingdom was one-way; they were only allowed to enter Secret Silver City from the outside but not allowed to return easily from Secret Silver City. Moreover, the portal was connected to a portal under the Tower of Heaven.
Every arcanists, alchemy warlocks, Trolls Blood Warlocks and so on, who entered the Secret Silver City, were subject to intense scrutiny to ensure that the coordinates of the Secret Silver City and the new world weren't leaked.
As long as the coordinates weren't leaked, it was impossible for the other three worlds to find that new world in the vast Astral World.
