What is the free city of Germany?

The Concept of Free Imperial Cities

The phrase freie Reichsstadt, or Free Imperial City, is often used interchangeably with the term Reichsstadt. However, it specifically refers to just seven cities: Basel, Strasbourg (Strassburg), Speyer, Worms, Mainz, Cologne, and Regensburg. These cities achieved independence from ecclesiastical rulers, thus securing their unique status...