Castillo de San Marcos

Castillo de San Marcos stands on a hill above Sanlúcar de Guadiana, a small town in the province of Huelva, right on the border with Portugal. From its ramparts on the eastern edge of town, the fortress overlooks the Guadiana River and the Portuguese village of Alcoutim on the opposite bank.

What is the history behind the castle?

The first fortification on this site dates to the 14th century, built to guard the "Raya del Guadiana" — the border line separating the old kingdoms of Castile and Portugal. By the late 15th century the structure had fallen into poor condition, prompting a major mid-16th-century rebuild that added the cylindrical towers still visible today, designed to better withstand artillery.

The castle's most dramatic episode came during the Portuguese Restoration War (1640–1668). Portuguese forces bombarded one of the towers to breach the defenses and, despite Spanish resistance, captured the fortress. It was only returned to the Spanish crown under the 1668 peace treaty.

After the Peninsular Wars in the early 19th century, the fort lost its military purpose and fell into disrepair. In 1866 the Spanish Ministry of War handed it over to the town council of Sanlúcar de Guadiana. During the 20th century it served for a time as a Guardia Civil barracks before being finally abandoned in the late 1950s.

What can you see there today?

In 2011, Spain's Ministry of Culture and the Regional Government of Andalusia funded a full restoration costing 2.7 million euros. The project reinforced the defensive walls and recovered the moat and glacis surrounding the fortress. Inside the early 18th-century troop barracks, an Interpretation Center now explains the history of the fortifications along the Spanish-Portuguese border — the best starting point for understanding the castle's centuries-long role as a border guardian.

Walking the walls, look for the Renaissance-era cylindrical towers and take in the sightlines that once let defenders control river crossings and monitor movement toward the port of Sanlúcar. The terraces offer one of the finest views in the region: the river, the valley, and Alcoutim across the water, all visible at once.

How do you get there?

Sanlúcar de Guadiana sits well inland in the province of Huelva, right on the Portuguese border. A car is the most practical way to arrive, since the town lies off the main coastal routes of the Costa de la Luz — which also means the castle sees far fewer visitors than other fortresses in the region. The site itself sits on a hill at the edge of town, within easy walking distance of the centre.

When is the best time to visit?

Since the site offers little shade and sits fully exposed to the sun, spring and autumn are the most comfortable seasons to visit, when inland Andalusian temperatures are milder than in high summer. Morning visits also give the best light for photographing the Guadiana valley and neighbouring Alcoutim.

Frequently asked questions

Can I combine a visit to the castle with a trip to Portugal? Yes — a small passenger ferry runs from Sanlúcar de Guadiana across the river to Portuguese Alcoutim, making it easy to pair the castle visit with a short crossing into Portugal.

Is there a museum inside the castle? The early 18th-century troop barracks now house an Interpretation Center dedicated to the border fortifications, tracing the region's defensive history over the centuries.

Is entry to the castle free? Access rules and any admission fees are worth checking shortly before your visit, since they can vary by season and by cultural events held on site.

Why was the castle built in this exact spot? The hill above Sanlúcar de Guadiana gave full command of the Guadiana river crossing and of movement between Spain and Portugal, making it a key defensive point on a long-contested border between the two kingdoms.