Together with her spouse she established the Santa Hermandad, instituted the Spanish Inquisition and completed the conquest of Granada.  
Although made heir to the throne in the Pact of Guisando this monarch had to fight a Civil War with Juana la Beltraneja in order to secure the throne.  The second child of Juan II and the successor to a half-brother Enrique IV, this monarch was succeeded by her daughter Juana "the Mad" while another daughter became the first wife of Henry VIII.  Together with her spouse she established the Santa Hermandad, instituted the Spanish Inquisition and completed the conquest of Granada.  FTP, name this Spanish monarch who financed Columbus's voyages and unified the Spanish monarchy with her husband Ferdinand.
Although made heir to the throne in the Pact of Guisando this monarch had to fight a Civil War with Juana la Beltraneja in order to secure the throne.  
Isabella of Castile or Isabella I or Isabella "The Catholic"