The town on the Neckar River was a “Free Imperial City” during the Holy Roman Empire, a status which put it in a league with many of Germany's most powerful cities, including Frankfurt, Cologne, Hamburg and Nuremberg. The city was a major base for the Teutonic Knights from the Middle Ages all the way up to 1805. The famous knight Götz von Berlichingen was held prisoner in the Bollwerksturm (Tower of the Bastion) from 1519 to 1522. While that tower still stands today, much of the city's historical architecture was destroyed when Heilbronn was carpet bombed by allied bombers in 1944. The town's nickname “Käthchenstadt” derives from a famous play called “Das Käthchen von Heilbronn” (Kate of Heilbronn) by Heinrich von Kleist.