What is special about the wall surrounding Antaris in the novel, The Tenth Power?

Antaris is surrounded by the Great Wall, which is made of ice. The wall possesses magical powers of its own so it is impenetrable by outsiders. The sisters sign a chantment when they need or want to open a gap in the wall to allow people in or out. It is in the Great Wall of Ice that Tamen, the High Priestess, encases the dead bodies of the sisters.