Alexia was locked into the dungeon of this castle along with the monsters. (Keep in mind that it takes several years for the saint to fully become the witch.) Idea was for the monsters to kill her since, since she's not the witch yet. Or backup plan: She's locked in a dungeon and can't escape. But what happened was that the monsters immediately accepted her and so didn't attack. Then later, when she became the witch, they helped her escape. (Keep in mind that both witch and saint are invulnerable to all but each other's powers, and neither can die of natural causes. Monsters are related to the witch, and so can kill the saint.)