Symbolism, Imagery, Allegory
The Tower Room is the part of Strachan's Ghyll that represents all of Cassandra's magical power. When she's at the height of her power, she's constantly going into that room to cast spells. She does things like summon Grisini and Clara from up there, and she even manages to keep the children from leaving Strachan's Ghyll when they figure out her devious plan:
"It's the maze. On the Tower Room floor—the red lines that woz painted there! We been making the lines wiv our tracks—over and over—"
"It's a spell," Lizzie Rose said despairingly. "We can't leave. She cast a spell on us so we can't leave." (43.32-33)
At the end of the book, though, Clara destroys the phoenix-stone, which in turn takes away all of Cassandra's powers. When this happens, the Tower Room crumbles to the ground, forever lost, just like Cassandra's power.