@CWO:
Just to toss an extra factor into this discussion, one should remember that (in real life) deploying a defensive minefield around a port isn’t consequence-free for the defender. Such minefields help protect the port from penetration by enemy ships, but they also impair the port’s capacity to handle friendly ships because they have to waste time picking their way through the minefields, something which requires good minefield charts, slow speeds and careful navigation by the harbour pilots.
You are completely right about this, but as it is ports have no restrictions on use. A naval base could service 100 carriers as easily as a single submarine. Unless youre suggesting that friendly ships should roll for mines, I am not sure that this is practical. Perhaps the conversion of a peacetime harbor to a mine-infested, artillery-ridden military base should come with a restiction on the number of ships that can use its facilities a turn. Maybe 8? That begs the inclusion of a smaller piece (port? harbor?) that has the same capabilities of the naval base, but on a smaller scale, and with less cost.