Most cities in Europe are granted right to self-government by their King. Some are granted more rights than others, but the majority swear fealty to a King. However, when a strong and self-sufficient city exists in the domain of a weak or distracted King, it may endeavor to cast off the bonds of fealty to stand on its own. Those cities that are successful are called Free Cities, or City-States. Most often these cities are major trade hubs, as it is much easier to be self-sufficient when connected to the trading networks of the rivers and seas.
It is the rare King that will allow a city to break free from his realm without making every possible attempt to stop it. However, if a King believes that he can benefit from a free city, such as through tariffs or through its acting as a barrier against an enemy state, he may decide that it is better to allow the city to stand on its own.