The walled city of Mertola is guarded by a stone fortress and overlooks the River Guadiana in the Alentejo, Portugal. The walled city of Mertola is guarded by a stone fortress and overlooks the River Guadiana in the Alentejo, Portugal.The walled city of Mertola is guarded by a stone fortress and overlooks the River Guadiana in the Alentejo, Portugal.