Why one ship should not seal a defended port

The game allows a lone enemy vessel to blockade a port containing a full Carthaginian fleet. That makes little operational sense because the force inside should be able to challenge such a token presence. The rule closes the port without modelling the force needed to contain or defeat the fleet inside. When Rome is blockaded later, its importance as a source of income prompts an immediate response. The weak operational model matters because a blockade can cut one of the campaign's major income sources.