Mértola Castle lies in the village of Mértola in the Alentejo province in Portugal. It is locally known as Castelo de Mértola.
The village of Mértola already existed in Roman times as Myrtilis. In the 7th century Mirtolah, as it was then called, was in the hands of the Moors. They probably built the first castle at this site, maybe on Roman foundations. Little is known of the role that Mértola Castle played in the medieval campaigns, except that in 1238 the castle was taken from the Moors by King Sancho II. He donated the castle to the Order of Santiago. They owned the castle for about a hundred years. The keep was built in 1292 during the reign of King João Fernandes.
The castle is built on the highest part of the rocky outcrop on which the village is situated. The castle defended the confluence of the Guadiana and the Oeiras river. It is also connected to, and therefore part of, the medieval city walls. The castle has a roughly rectangular groundplan of 40 by 20 meters. There is a cistern in the middle of the bailey.
The castle can be visited for a small fee. You can climb the keep to the top, via wall stairs, which offers splendid views of the surrounding lands and the Guadiana river valley. The village itself is also very picturesque.