The castle was isolated. The array surrounding it was extremely powerful. Even though the Stellaris Sect's battleships were inferior to the Federation's—many of them being old goods that had been discarded by the Federation—there was no doubt that their power was extremely terrifying.
With a single blast, even a mountain would instantly turn into nothingness. There were not many barriers that could block such power.
The Monster Cult definitely had not come up with this plan at the spur of the moment. They must have planned it beforehand. All the universities and sects, including the Stellaris Sect, had been plotted against by the cult.
Although the other overseas sects and aristocratic families, including the Stellaris Sect, had conflicts with the Federation, they were also cooperating and resisting the cult.