The Interior Castle written in the sixteenth-century is a masterpiece of spiritual literature. This mystic book is as inspirational and enlightening today as it was when it was first written. Overcome one day by a mystical vision of a crystal castle with seven chambers, each representing a different stage in spiritual development, Teresa immediately wrote 'The Interior Castle'.

In her most important and widely read book, St. Teresa describes how, upon entering the castle through prayer and meditation, the human spirit experiences humility, detachment, suffering, and, ultimately, self-knowledge, as it roams from room to room. As the soul progresses further toward the center of the castle, it comes closer to achieving ineffable and perfect peace, and, finally, a divine communion with God.

St. Teresa was a reformer of the Carmelite Order and is considered to be, along with Saint John of the Cross, a founder of the Discalced Carmelites. She became the first female to be named a Doctor of the Church, in 1970, and is one of only three females to be awarded that honour.