The Saint-Gral Labyrinth was built by Crimson Roselia as a way to train and test the witches that wanted to leave the village and become wandering witches. The inside of the labyrinth can change between several phase spaces depending on the level of training required.
In August of S.1206, Class VII undergoes a rehabilitation program under Roselia's tutelage to regain their previous combat capabilities and endurance after two weeks of resting following the events at the Gral of Erebos. With the old Class VII having recovered earlier, Juna, Kurt and Altina, together with Randy, make their way through the labyrinth before setting off to find the singularities needed to locate the Black Workshop. Roselia herself is the final test of their first day in the labyrinth, unleashing her original form.
As they find more singularities, higher phases become available for usage, but not required.