A circle moght actually be a reasonable shape for a city-state enclave within a larger nation. I'm thinking of a city which starts out in the medieval era as gaining a degree of independence through exemption from taxation by the lord of the surroudning countryside. This exemption applies to "all land within thw city walls" and a precedent is set up that if the city buys up surrounding land and extends its walls the tax emeption is also extended. Eventually the city and surrounding feudal domain become separated nations, but the idea that the city can expand indefinitely through buying up land threatens the surrounding nation, especially as the industrial revolution brings greater urbanization. Eventually, the two neighbours agree upon a limit to urban expansion and this limit is set as a circle a certain radius from the centre of the city.