The actual chateaux seems to be some miles from the sea so I figured the port was along an old river course. There are currently two nearby waterways that may be navigable and one may be descended from the one that was associated with the old port.
I’m sure there has been silting (my glasses need cleaning, my typing looks fuzzy) but when the sealevels were higher, the estuaries were deeper and broader and reached further inland. Rivers and even what look like small streams today worked pretty well for the smaller ships of the time.
The lack of deepwater harbors in western Europe was a problem for the Allies during WWII after DDay, for example.
I wonder if mapping the medieval shoreline and the extents of estuaries on the eastern seaboard of N America would be a worthwhile first step for looking for ProColumbian Scandinavian landfalls, and for earlier Roman Warming Period Celtic landfalls, and for the much much earlier Red Paint maritime culture.