What geographical feature allowed the greek poleis to develop independently of one another?

Poleis is the plural of polis, a city, so the question is about the ancient Greek cities. 

The geographical feature was the whole geography of the Greek territory, which divided the land into easily defended but not easily attacked places: the islands and the valleys - the Greek peninsula has many islands which separate the valleys, and there are many islands along its shores. 

 The geographical feature that allowed the Greek poleis to develop independently of one another was the whole geography of the Greek territory. This divided the land into easily attacked but not easily defended places: the valleys and the islands. The Greek peninsula owns many islands which are separated from the valleys.