The Interior Castle, also known as the Book of Mansions, is considered Teresa of Avila's greatest and most mature explanation of the spiritual journey. For Teresa, growing spiritually is traveling inwards to the centre of our being where God dwells, yet too few set out with resolution to reach the Divine Presence. The Interior Castle treats of the spiritual journey in a totally original way. Here Teresa is not theorizing from books; instead she writes and speaks from personal experience and close observation of others. The book, in fact, was written with her nuns in mind, who were neither theologians nor particularly well educated, some indeed were illiterate. In those days there was little teaching available for women, and the manuscript was initially intended for reading aloud in community. As always Teresa stresses that prayer and life are interlinked, and that writing about prayer must have its parallel influence on daily life: 'We pray as we live and live as we pray.' God and neighbour, for Teresa, are inseparable, and she sees the spiritual journey as one of friendship with God and with those we live with.
It means standing and living in the truth, whether in prayer or in life. This, she stresses, is the way to maturity and true inner peace.