secret area?

[nach]

While walking in man-made dungeon I just got the following yellow message:
"Because the shield stone power of [ Plain ] weakened, the secret area is being opened."
what does that mean? Never had this message before. Search function says there are secret areas in snow war, was the message related to that? I don't know if snow war is currently running since I don't know the schedule and my char is way over lvl 60

[Tensu]

"Because the shield stone power of [ Plain ] weakened, the secret area is being opened."

this means one of the castle wars is about the start (in this case it's Plain warfield)