When I play as the Romani I always blockade the port. If I for example besiege Syrakousai which was a port city then I have to blockade it with a fleet, but if I besiege Rhegion I´m confined with simply sending two units to the port that I´m pretty certain is supposed to represent Croton, thereby representing that those units have occupied Croton. Of course Croton wouldn´t just open its gates unless things were really going the Romans way, so I don´t send those units there until I´m already besieging Rhegion and is just overall controlling the area.