'The Interior Castle' is a classic of Christian mysticism, written with some reluctance by its author, St Teresa of Avila. The saint spent most of her life as a Carmelite nun, and was noted for her piety and the frequency of her visions. In 1577 she was instructed by her superiors to produce a work on prayer for her sisters in the order. The result was a book of great spiritual significance, in which she wrote of her vision of the human being as a crystal globe, containing seven mansions. It is through these that the soul must make a progressive pilgrimage, to final union with God in the seventh mansion. St Teresa describes the prayers and meditations for this spiritual journey in great detail, and also warns of the obstructions and barriers that the Devil erects to prevent passage into the various mansions. Full of encouragement and advice for the modern aspirant, 'The Interior Castle' is as relevant today as when it was first written over four hundred years ago.