question: The queen of Castile, Joanna of Castile, married this member of the house of Habsburg who was the son of which monarch?
answer: Holy Roman Emperor Maximilian I
context: Title: Isabella of Aragon, Queen of Portugal
  Passage: Isabella, Princess of Asturias (2 October 1470 – 23 August 1498) was a Queen consort of Portugal and heir presumptive of King Ferdinand II of Aragon and Queen Isabella I of Castile, as their eldest daughter.  Her younger sisters were Catherine, Queen of England, Queen Joanna I of Castile, and Maria, Queen of Portugal.
  Title: Philip I of Castile
  Passage: Philip I (22 July 1478 – 25 September 1506) called the Handsome or the Fair, was the first member of the house of Habsburg to be King of Castile.  The son of Holy Roman Emperor Maximilian I, at the age of three Philip inherited the greater part of the Duchy of Burgundy and the Burgundian Netherlands (as Philip IV) from his mother, Mary, and at 27 briefly succeeded to the Crown of Castile as the husband of Queen Joanna, who was also heir presumptive to the Crown of Aragon.  He was the first Habsburg monarch in Spain.  He never inherited his father's territories or became Holy Roman Emperor because he predeceased his father, but his son Emperor Charles V eventually united the Habsburg, Burgundian, Castilian, and Aragonese inheritances.
  Title: Joanna of Castile
  Passage: Joanna (6 November 1479 – 12 April 1555), called the Mad (Spanish: "Juana la Loca" ), was queen of Castile from 1504 and of Aragon from 1516.  From the union of these two crowns modern Spain evolved.  Joanna married Philip the Handsome on 20 October 1496.  Philip was crowned King of Castile in 1506, initiating the rule of the Habsburgs in Spain.  After Philip's death that same year, Joanna was deemed mentally ill and was confined to a nunnery for the rest of her life.  Though she remained the legal queen of Castile throughout this time, her father, Ferdinand II of Aragon, was regent until his death, when she inherited his kingdom as well.  From 1516, her son, Charles I, ruled as king, while she nominally remained co-monarch.
  Title: Constance of Burgundy
  Passage: Constance of Burgundy (8 May 1046 – 1093) was the daughter of Duke Robert I of Burgundy and Helie de Semur-en-Brionnais.  She was Queen consort of Castile and León by her marriage to Alfonso VI of León and Castile.  She was the granddaughter of King Robert II of France, the second monarch of the French Capetian dynasty.  She was the mother of Urraca of León, who succeeded her father in both Castile and León.
citations: Philip I of Castile Joanna of Castile