City-states have been important in the developed of western civilization and culture from the classic Greek variety to those that encompassed and developed in Europe, especially those of Italian and German origin but others played important parts also. Following is a link to a good synopsis on some of the more important city states in Europe. A history book I am reading went into the City-state of Venice and it was interesting how the governing body was structured. This piqued my interest in this subject. City-States seemed to be sources of great innovation and experimentation: in economics, political structure, trade, ship building and general manufacturing, banking, and culture that contributed much to European Civilization. See link below:
From above link, the last paragraph:
In 1500 the city-states played essential roles in European politics, economy, and culture. But they could not afford the money and manpower to defend themselves against aggressive territorial monarchies and princedoms. They could not compete against national economies. And with the exception of Venice, their artistic and intellectual greatness faded. The city-states were major losers in the centuries between the Renaissance and the French Revolution.
The above is particular to the city-states of a set period. Obviously many other city-states have existed, in different parts of the globe, and prospered and declined, been conquered, an/or morphed into other political structures.