hello, can you link your map again? I'd very much like to see it
I would suggest a few more places in Portugal.
Valença (North, next to the border with Spain) is a small town with a lovely historical center completely walled. Inside the fortress, that is in very good shape, there are many buildings and several open areas that could be used for farming.
Óbidos (Central Portugal, just 10Km from the ocean) is a smaller town fully inside a castle. Also a good place, and not so far from the sea. They could create a cleansed area to allow fishing and farming there while living inside the castle.
Évora (Central, a bit South, closer to Spain) is my preferred. The old city center is fully enclosed by the old walls. There are just a few points where those walls were demolished to let streets pass, but those would be easily blocked. The inside is very large and could hold many thousands. The only limitation would be food. I imagine several buildings being replaced by open areas or converted in greenhouses. Oh, and it has the ruins of a roman temple (to Diana
https://en.wikipedia.org/wiki/Diana_(mythology)) in the center, so may be a good place for a renewed worship on the roman gods...
And last but not least
Sagres. That's not a town, but a fortress in the Southwest corner of Portugal. It's a promontory that advances into the sea, and the walls simply close the connecting point. Very few constructions, and a very exposed area, but what makes it promising for a story are the legends that consider it a magical place, where centuries ago people performed rituals. That large circle of stones, of unknown function, is the oldest remain found there. Some say it had astronomical meaning, others a magical one... All that, together with the spectacular setting, makes it a nice candidate.
Note 1: sorry for not finding better videos.
Note 2: it may seem like that, but this post is not sponsored by the Portuguese tourism authority...