This book offers an integrated study of the English princess and Castilian queen Catherine of Lancaster (1373–1418), drawing on available archival, architectural, and poetic sources in England and Spain. Catherine’s mother, Queen of Castile in exile, and father, the powerful military commander John of Gaunt, raised her to take the Castilian throne. This volume connects Catherine’s early life, providing insights into those who promoted her cause from birth as Princess of Castile, and her later life as Princess of Asturias, then Queen-consort, and finally Dowager and Co-regent of Castile. Her influence on the Castilian court’s poetic circles has not previously been connected to her English heritage. Poetry written about her and influenced by her was compiled into a songbook presented to her son, Juan II. The book brings new understanding of the role an Englishwoman played in Trastámara Castile’s turbulent history.