In The Interior Castle, the 16th-century mystic and reformer Saint Teresa of Avila presents a profound spiritual guide describing the soul as a castle with seven mansions, each representing a stage of deepening union with God. Drawing on her own mystical experiences, Teresa explores prayer, self-knowledge, humility, and divine grace, offering practical advice for those seeking inner transformation. Written with poetic beauty and theological insight, this work remains a cornerstone of Christian mysticism and Catholic spirituality, guiding readers through the journey from sin and distraction to intimate communion with God. A timeless manual for contemplative life.