It is a common trope in fairy tales. Though how often did it happen historically? Was it anything like in fairy tales if it did happen?
By - Bluecomments
Extremely rare. Most of the time princesses didn't marry for love, but for politics. If they were important enough to be a hostage, they were likely too important to marry a knight sent to rescue them.
Very uncommon or nonexistent. Royal women were nearly always married for political purposes, and as far as I know never as a reward to a knight errant.
In the 1700s Princess Maria Clementina Sobieska was rescued by Charles Wogan and three kinsmen, Richard Gaydon, Captain Missett, and Ensign Edward O'Toole. But although they were rewarded, none of them married the princess.
In the 1100s Queen Eleanor of Aquitaine escaped an ambush in part due to the efforts of William Marshal, a young knight, who was captured and held for ransom. The Queen paid his ransom and he served her for some time, but Eleanor was already married.
Pretty much never -- though there've been enough princesses and knights that it's happened a few times just by dumb luck.
The one example that comes to mind is Sir Peter of Dreux marrying Duchess Alix of Brittany. Not much rescuing involved though: Alix's dad (King Phillip II) was specifically looking to thread the line between someone who was good enough for his daughter, but also enough of a nobody that he didn't need to worry about having another backstabbing noble in his court.
Peter, for his part, just kept on being a knight but with a fancier house to come home to. He fought in several crusades, won some battles, lost some battles, etc until he finally died in a shipwreck.
Peter of Dreux did it in 1200’s Britain. Although, it involved no rescuing