Volcano

On the way to Cartagena from Murcia, is the Puerto de la Cadena. An area of natural passage in medieval times was fitted to bring up travelers through these mountains, but by paying a toll.
In the image at the top of the mountain, you can see the castle Macher, or their remains. A castle of the thirteenth century, who built Ibn Mardanish "Wolf King" as fortress on this passage. It was built on Roman remains, so that its base is square. But the castle was not completed due to lack of funds.
You can climb through a trail. A good place for hiking.