Trying to build a coastal region of feudal/renaissance city states surrounded by small feudal kingdoms. Looking for resources that are valuable beyond their access to sea, its bounty and the trade that comes with it. Also looking for features beyond having big walls and/or being difficult to access.
For example I read somewhere that ancient Athens had access to a silver mine, which I assume helped them significantly.
Anything unique or a bit off the wall is very welcome, but nothing magical or unrealistic please.