Mertola extends over the two banks of the river Guadiana and covers an area of more
than 1200 square kilometres.
Before the Roman period, Mertola was already a significant commercial axis, because it was the most Northern inland port on the river. It is here that people arrived and settled and that goods that came from the most remote regions of the ancient Mediterranean world circulated.
It is also here that the river and the roads which brought the bread and the olive oil from the clayey soil of Beja and minerals of Aljustrel and Sao Domingos crossed. The combination of all these factors made Mertola an historically important town. The monuments and landmarks of this past are scattered over the city, still surrounded by the old wall, a kilometre long.

Archaeological excavations, which initially concentrated on the grounds of the old citadel, were later extended to other parts of the city.
The first site uncovered part of the old Roman forum, revealing the ruins of an immense gallery.
In the same place, a quarter built on an old Christian baptistry in the 12th century, is believed to have been occupied until the middle of the 13th century.
In the basement of the Town hall and in Rossio do Carmo, ruins of a Roman house and a basilica, constructed in the 5th century, were discovered. In this last site, it was possible to identify the ruins of Roman , Christian and Islamic burial places.
More recently, excavations undertaken near the city have uncovered the ruins of the Saint Sebastian chapel (16th century), which was built on a roman necropolis..