The son of which German emperor was a prisoner of the Italian city of Bologna for almost 23 years in the 13th century?

King Enzio of Sardinia spent the last 23 years of his life as a prisoner of Bologna from 1249 to 1272.
Enzio (from the German name Heinrich, sometimes rendered as Enzo) was an illegitimate son of Emperor Frederick II of Hohenstaufen and by all accounts his favorite. They shared a love for falconry and poetry among others, and Frederick II valued his son as a capable leader and commander.

Enzio was 24 when he was captured at the Battle of Fossalta during a battle with the Guelphs, a powerful faction in northern Italy opposed to the emperor of the Holy Roman Empire. Frederick II repeatedly demanded the release of his son, but to no avail.