St. Teresa of Avila (1515 - 1582) was a mystic Carmelite nun. She composed the Interior Castle as a spiritual guide to union with God. She claimed to have received a vision from God where she saw a crystal globe with seven mansions, and God was in the innermost mansion. St. Teresa understood this vision to be an allegory of the soul's relationship with God, with each mansion representing a step on the way towards the "spiritual marriage" or union with God, the seventh mansion. This journey starts with prayer and meditation.
She also relates the obstacles the Devil puts in various rooms, to stop believers from having union with God. Throughout the entire work she gives encouragement and advice for spiritual development.

In 1622, forty years after her death, St. Teresa was canonized by Pope Gregory XV, and, in 1970, she was named a Doctor of the Church by Pope Paul VI. She also wrote "Way of Perfection."