Although the Grotto Heaven is independent from the Longevity Realm...in reality, there is no spatial membrane; rather, within the Grotto Heaven, there exists a set of independent world rules.
In other words, the people from Yiling Town and the Divine Sword Sect, as well as people outside, can enter the Grotto Heaven...without any impediment.
Unless Li Qian moves the Grotto Heaven...only then would unusual changes occur, forming a self-protective boundary membrane.
Once the Grotto Heaven reattaches to the big world, this boundary membrane will vanish again.
However, this is under the premise that Li Qian does not interfere.
If he does not want certain people to enter the Grotto Heaven, he can set restrictions, no matter how powerful the person is, they can forget about entering the Grotto Heaven.
"The Grotto Heaven is quite impressive."
