Why did many Portuguese writers use both Portuguese and Castilian after 1450?

After 1450, many Portuguese writers utilized both Portuguese and Castilian due to the close political and cultural ties between Portugal and Spain. These connections were frequently reinforced by dynastic marriages, which often fostered a predominantly Spanish atmosphere at the court in Lisbon.1

During this period, Castilian literature served as a significant model for Portuguese court poetry and theater. The influence of Castilian became particularly pronounced in the mid-15th century following the decline of popular troubadour poetry, with figures such as Pedro the Constable initiating a trend of writing in Castilian. This bilingual practice persisted for more than two centuries, and many notable Portuguese writers—such as Gil Vicente, Jorge de Montemayor, and Francisco Manuel de Melo—produced works that are considered classics of both Portuguese and Spanish letters.123