What is Bluebeard's Chamber in The Blue Castle: A Novel by L.M. Montgomery?

Bluebeard's Chamber is a small shanty on Barney's property, it serves as his own special place... his man cave. Valancy is forbidden to enter the chamber and adheres to her husband's wishes. One day, she needs a pen and enters Bluebeard's chamber to retrieve one.... to her surprise, she finds out the man cave is the place Barney writes.