Castle Of Mertola (Castelo De Mértola)

The castle of Mértola sits on a rocky spur at the top of the old town, above the point where the Oeiras stream meets the Guadiana in the Baixo Alentejo. The position is as important as the building. From the walls you see the two rivers, the white houses tumbling down the slope, and the dry hills of one of the emptiest parts of Portugal, which makes this one of the best viewpoints in the south of the country.

The hill has been occupied for a very long time. Mértola was a Roman river port, with the Guadiana carrying ore from the nearby mines of the Iberian Pyrite Belt, and it later became an important town under Muslim rule. The defences visible today have layers of different periods. The fortress was strengthened under Islamic rulers in the medieval period, and the town was taken by Christian forces in 1238, in the reign of Sancho II. It was then handed to the Order of Santiago, which rebuilt the castle at the end of the 13th century and added the keep that dominates the skyline.

The plan is roughly rectangular, about 40 metres by 20, built of local stone, with a courtyard inside the walls and a large cistern that stored water for the garrison. The keep is the main feature, a tall square tower that you can climb for views over the valley. The walls are plain but impressive, and the ramparts are open to walk along. Notice how the stone changes colour in different sections, a sign of repairs across the centuries.

Today the castle is looked after by the local council and forms part of Mértola's identity as a vila museu, a town where the archaeological and historical sites are open to the public as one museum. The Moorish mosque, now the parish church, is just below the castle, and the remains of Roman buildings such as the cryptoporticus, the Torre do Rio and parts of the old streets are within easy walking distance. The Islamic Art Museum is also nearby. A visit to the castle fits naturally into a walk through the old town's cobbled, whitewashed lanes.

Wear sturdy shoes, because the climb up from the river is steep, and bring a hat in summer, since the castle has little shade and temperatures in the Alentejo can be extreme. Spring is lovely, with green fields and flowers, and autumn is mild. Opening times are restricted to working hours, so it is worth checking the schedule locally before you visit. The castle suits history lovers and photographers, and it is one of those places where a modest building is made memorable by its setting.