Considering the current state of implementation of natural harbors, I'd like to append
@Koiodoros his post for Flanders natural ports and Northern France
River Ports:
Brugge - despite heavy siltation past the 1430s, Bruges always retained its coastal connection through a canal to Sluis. Although I agree Bruges should decline, Sluis, also within the location's borders, retained its river port position for much longer, as evident from
Pieter Pourbus' map (1571) and the
Frickx map (1712).
Veurne -
Nieuwpoort was established in 1163 at the mouth of the Yser river and was an
important river port.
Tancarville -
Le Havre had a Coastal barrier system with plenty of protected gullies. Neighboring Harfleur also had a
significant port connecting to Le Havre through a canal.
Abbeville - developed in the 11th century as the capital of Ponthieu and
was a river port on the Somme river. The mouth of the Somme is situated in a Bay, while the port towns of
Le Crotoy and
Saint-Valery never became huge, they were strategically interesting for the French, English and Burgundians in their conflicts.
Bay:
I first considered Ostend to be a bay, with the Testerep coastal barrier island, but as it was destroyed by a storm surge in 1394, I think 'flat coast' remains the more approriate option.
unaltered screenshot of current harbor implementation
View attachment 1241841