We were walking on the beach in Cadiz and saw this castle or fortress at the distance. We stopped in to get more information. It is called Santa Catalina. Not much information was provided in English but here is what we leaned. It is an old fortress used in different wars , including more recently WW2. Entrance is free but there is not much to see. Several rooms are being used for temporary exhibition. You can see star shaped walls of the fortress and that’s about it. We didn’t know what to expect and it was free so we didn’t complain.