The original ruleset has coastal defenses with gunpowder

[wieder]

This was proposed at an earlier time and might make sense from gameplay point of view. At least if gunpowder is made even more expensive as planned.

Also, since there was air defense before missiles, it might make sense to create some kind of anti-aircraft unit for defending the cities before SAM batteries. Maybe a unit that gets defense bonus against aircrafts, like AEGIS cruiser, but would also have a higher cost compared to normal city defending unit.