“Castelo da Foz” is also known as “John the Baptist Fort”. It was built in the end of the 16th century for the defense of the Atlantic coast and the entrance to the Douro River. The building of the castle absorbed the Church (in renaissance style) that was previously in the same place.
There is no ticket required to visit the castle but the place is not crowded at all, there was only one other tourist when I visited, no one else!
There was a painting/art exhibition in a large room inside, the only person there was the artist himself. He confirmed me that one can explore the Castle at his own will but there are some rooms off-limits where apparently there are some administrative offices.
From the balcony outside it is visible that some parts of the Castle have suffered the passage of time. There’s a good view of the mouth of the Douro River.