In Oporto one can’t find the castle itself which seems strange in such an old city. The Romans referred to a population center along the Douro river as Portuscale , on the hill of Pena Ventosa which is where the Cathedral is  today. Here we have many Roman and pre Roman vestiges which suggests a continuous occupation, so there was suppose to be some defensive building

But there was a castle or not ? What we find today are the remains of two defensive walls , both medieval !

The old walls existed in 1120 by the time of donation of the borough to the Bishop, the  document mentions that along with the village,  extra land outside the walls also belonged to him.

Regarding the new walls were imagined around  1336, during D.Afonso IV , after  the invasion attempted of the Spanish King Afonso XI. Are known as the Fernandina´s for were built during D.Fernando´s reign.

What remains today to be seen is considered a national monument.