Though most likely apocryphal, this musical tale is worth retelling:
Shortly before Christmas 1192 Richard I was captured near Vienna by Leopold V, Duke of Austria, who accused the English monarch of arranging the murder of his cousin Conrad of Montferrat. Moreover, Richard had personally insulted Leopold by casting down his standard from the walls of Acre during the Third Crusade. Duke Leopold kept him prisoner at Dürnstein Castle.
At first no one in England knew where their king was. A minstrel called Blondel searched for his master throughout Europe in vain. Returning home through Austria he learnt there was a closely guarded prisoner whose identity was a secret nearby. Suspecting it could be his master, he located a tiny barred window high up on the castle wall which he thought could be a cell. Under the window he sang the first couplet of a Troubadour's song which he had composed, a voice responded with the second couplet. It was the King.
After being found by Blondel, Richard was handed over to Henry VI, Holy Roman Emperor. It was while languishing in captivity as the Holy Roman Emperor's prisoner that the monarch wrote this song as an address to Marie of France, Countess of Champagne. Eventually, the English people paid a huge ransom to set him free.
(Source The Encyclopedia of Trivia