What are the factors that would determine whether a city on the coast will become a bustling metropolis or whether it will always remain a modest fisher's village?
- A good natural port
Sailing ships and oared galleys are hard to maneuver. The usual wind directions and the available shipping channels must allow ships to get in and out, most of the time. Calm waters inside, good ground for anchors, beaches while there are still galleys to be beached.
- Healthy climate and supplies
A ship comes in and needs good water and food. They don't want a malaria hellhole if there is any choice.
- Good governance
It doesn't have to be democracy and the rule of law as we know it, but there has to be a stable, predictable environment for business.
- Access to land consumers and producers
It also helps if the port is the "natural" loading and unloading point for an agricultural area or large city.
But it doesn't have to be all of those:
If there are many traders to trade with, more traders will come. So a tiny initial difference can grow decade by decade, century by century.
Does it have good transportation links with the rest of the land? (This is mandatory and cannot be waived.)
Does it have a natural port?
Is there an active economy behind it to be served by the port?
If it doesn't have a natural port, then is there a very active economy behind it which would justify building an artificial port?
(If you do not need a "bustling metropolis" but are satisfied with a moderately successful city, then the requirement for an active economy behind the wannabe port can be replaced with having a strategic position, where ships must pass.)
(Also, note that a great port does not have to be on the coast; for example, neither Antwerp nor Hamburg are on the coast, and this does not prevent them for being among the largest and busiest ports in Europe. What's important is that sea-going ships are able to come to the port, for example on a river or on a canal.)