She was 62 (1577) when wrote in less than 6 months, her book called ‘The Mansions‘ or ‘The Inner Castle‘ in obedience to an assignment given by her spiritual leaders. The first paragraph is her complaining about that. Later she saidd got the vision of a crystal globe with 7 mansions (& even more mansions) with God in the center.
The book is a guide to spiritual purification, describing the soul as a castle with 7 mansions, on the path to a mystical union/marriage with the divine ‘Majesty’ in the center or innermost mansion. She wrote that by emptying oneself of all attachments to creatures, God then comes and fills the soul with Himself, but this is Catholic doctrine about sin & purgatory, CCC #1472. At the end of the journey the soul would reach Spiritual Marriage in the 7th Mansion, the souls is free from sin & have union with God.