The Forte de Sao Joao Baptista da Foz, is also known as the Castle of Sao Joao da Foz, and overlooks the Douro River, where it meets the Atlantic Ocean.
The fort stands in a state of disrepair at this point, but it does seem like some restoration is being planned.
The history of the building is somewhat tainted as it overlooks a clay tennis court.
Although this is not a main tourist attraction, it is worth taking a look if you are in the area. The beach across the street from the fort is also good for families. Retaining walls have been built, so the water is rather calm in some spots, which is a good place for kids to swim. The water does get rough when walking on the jetty/pier however, especially when windy as water was blowing onto the jetty.