St. Teresa of Avila made an enormous contribution to Christians' understanding of contemplative prayer. She came up with a metaphor of a castle to help explain this type of non-verbal prayer. In her book Interior Castle, she compared a Christian's interior prayer life to a castle with many rooms, with each room representing a deeper level of prayer and union with God. This metaphor helped many in Christianity better understand contemplative prayer. St. Teresa believed that through silent prayer, Christians could progress through the rooms of the castle to achieve closer union with God within themselves.