Corinth

Ancient Corinth was a very rich city-state because all land traffic between northern and southern Greece had to pass through the isthmus of Corinth and they charged a tax. All sea traffic between the Gulf of Corinth and the Aegean Sea had to be carried over the isthmus, which also enriched the Corinthians. The Romans built on top of the earlier Greek ruins, so it's been hard to test my theory about political geography and citizenship.