This is a small part of Cartagena’s city wall.
In the 17th century Cartagena was in Spanish hands. Royalty decided to build a fortress with a wall around the city to protect it from pirates. It should take more than 208 (!) years to finish the 11km wall. In 1756, while in Madrid, Charles III of Spain took his spyglass and allegedly said: ” This is outrageous! For this prize those castles should be seen from here!”
They are nice walls, though.
Oliver, your visual flaneur