Well the concept was more defensive than anything. It’s not for chasing them down, but instead you put a CDS in every coastal city to ward off angry fleets. They would also have a range equal to or greater than cruiser, but this could be tipping the balance as an anti-ship unit out ranging cruiser sounds like it could have many underlying problemsCoastal guns would be useless, because navy players would just tick or missile them down.
Same goes with CDS/MRL, navy moves quicker than any ground unit, so they can easily out tick you any time.
RG are in theory anti-navy, but in practicality, lack of range makes them pretty useless, even more since units need to fully disembark, before they can shoot after air assault.
You could have 3 levels in rural provinces and 5 levels (just to keep it in line with other buildings) in cities, with each incremental improvement increasing the damage and range of the coastal guns. The top level would be at least equal to the longest range of any ship.
That way, you could surround an important city with coastal defences, both in the city and in the surrounding regions. Yes, a powerful fleet should be able to destroy them, but it would take a strong task force and significant time to achieve - thereby giving the defender time to organise other counter-measures.
Because they would be province improvements rather than units, they would be visible to all players at all times, even when under construction - that way, the naval attacker would know they were there and could take their own measures (like sabotage or aerial bombardment) to prepare the ground before sending in their ships.
It wouldn't be a fail-safe; just a way of buying time for the defender.