According to the Civ Editor Help Topics Menu, this field in the Rules under the "Improvements and Wonders" tab gives a city improvement the ability to "automatically attack enemy sea units". This is set to a value of 8 for coastal fortresses in the default game. Yet I don't think I have ever seen a coastal fortress attack an enemy vessel in any game I have played. Usually, in my games enemy vessels bombard a city and then sail away unscathed. Does this function actually work? :confused:

It's like zone of control. It's only if they pass.