1. Archaeologists have already been able to identify a river bank and a gutter on the site.
2. the Atlantic coastline is only a few kilometres away from the site
3. boats and ships could have transported merchandise and people to the chateau from England or even Spain
4. the stones that were brought to build the chateau come from four kilometres away, and transporting them here by boat would have been much easier.
5. a metal pilgrims medal has also been discovered, indicating a fishing trade and economic exchange.
It seems like they are postulating that the port was a RIVER port and the stones were transported up the river. Does the evidence of fishing trade suggest that ocean fishing vessels sent their catch up-river by boat? Or maybe some fisherman lived in this port town just lost his pilgrim's medal there? Fishermen certainly wouldn't commute to the coast several kilometers away for their daily work.
Or was this possibly an ocean coastal port and the ocean levels have dropped a lot in 1,000 years?
At first I was wondering what 1,000 year old Port would taste like.
The River changing course as it silts up is more likely.
The coastline of the time left "fossil" traces, so it's generally available info, but I dunno if that's been done here. That has worked in other places, like the east coast of England.