When Aiwass and Cardinal Galatea arrived at the Spirit Summoning Tower, they saw it already enveloped by a dim yellow barrier.
This must be the preservation spell set by the Lich Grunewald—currently, it seems that in terms of the level of preservation arts alone, Grunewald indeed falls short of the Amber Master.
However, with the support of the Spiritual Bead Sky Department Spine, the sturdiness of this barrier has already exceeded the energy limit of the Sixth Power Level in the material realm.
In the absence of a personal intervention by the Fallen Celestial Marshal, it is quite difficult for the Upper-Level Demons to breach this barrier.
It's not that it can't be opened... but it would be very laborious.
After all, the Path of Transcendence is inherently good at transcending conventions and breaking through limits.
If the demons initiated a ritual, they could at most break this insurmountable perfect barrier in a week.