Castillo de San Marcos, Medieval fortress in Sanlúcar de Guadiana, Spain.

Castillo de San Marcos stands on a hill east of Sanlúcar de Guadiana. Its mid-16th-century rebuild added low, thick cylindrical towers, some encasing earlier square medieval towers, and moved the entrance. Inside are a cistern, the governor's house and artillery ramps; a 1666 Portuguese siege added an outer bastion and half-moons.

Portuguese forces built the original fortress in the 13th century to control this border territory. Later, it came under the rule of the Kingdom of Castile as the kingdoms worked to define their frontier lands.

The fortress shapes the local identity and remains the most recognizable landmark of Sanlúcar de Guadiana for both residents and travelers. It stands as a focal point where people come to understand the shared border story and observe the lands across the water.

The castle stands on a hill east of the village, reached on foot by a steep climb from the centre. The restoration completed after 2011 consolidated the defensive structures and recovered the moat and glacis, so the grounds are walkable. The Interpretation Centre for the fortifications of the Portuguese border sits inside the early 18th-century troop barracks.

The fortress directly faces its Portuguese twin, Castelo de Alcoutim, across the Guadiana River, creating a physical reminder of medieval competition between two kingdoms. This neighbor relationship reveals how close the frontier was and how crucial this river valley proved for defense on both sides.