She was born in 1526 in Kraków, a princess of the house of Jagiellon — daughter of the King of Poland and of Bona Sforza, who carried the blood and the tastes of Renaissance Italy north with her. When she married Johan, Duke of Finland, in 1562 and came to Turku, she brought a world the grey castle had never held: Italian art, books and music, a Catholic and cosmopolitan refinement at the cold edge of the realm.
Her devotion was immediate and total. When Erik XIV besieged and imprisoned Johan in 1563, Catharina was offered free passage home to Poland, but she pointed to her wedding ring, engraved Nemo nisi mors, "nothing but death," and chose captivity at her husband's side. At Gripsholm she shared four years behind locked doors, and there, in 1566, she bore their son Sigismund: a child who would one day wear two crowns, born a prisoner.
When Johan took the crown in 1568, she rose with him as queen of Sweden. She died in 1583 — and the union her marriage had always pointed toward came to pass: that prison-born son, raised Catholic, would wear the crowns of both Poland and Sweden, the Italian and the Polish and the Swedish braided into a single heir.