Strapped into a harness, my hands clinging to a metal bar, I flew – at terrifying speed along 720 meters of zipline suspended above the Rio Guadiana – from Spain to Portugal.

I had taken a boat from Alcoutim (on the Algarve side of the river) to Sanlúcar de Guadiana (in Andalusia). From there I was driven to a launch pad on a rocky peak with breathtaking views of both countries. Did I scream? As I rushed to Portugal, I tried to focus on these views: two dazzling white villages, the wide green river below, a castle on one side, a castle the other. It was all over in less than a minute, but thanks to international time difference I saved an hour.

The LimiteZero experience (the world’s only cross-border zipline) was just one of the highlights of a winding road trip along the Rio Guadiana – the long river that originates in the Spanish province of Albacete, gliding over the Portuguese border near Elvas in the Alentejo and heads south to the Bay of Cadiz in the eastern Algarve.

LimiteZero, a unique cross-border zip line, covers Spain and Portugal. Photography: Luis Costa

Driving a rental Fiat 500, my husband Dave and I mostly stayed on the Portuguese side of the river, taking six days to do what amounts to a three and a half hour drive. For much of the way, the river forms a natural border between Spain and Portugal; a trail of castles and fortresses dazzle from the opposite shores. Too many castles for one trip, perhaps, but there’s plenty more to see: salt marshes, walled towns and ancient river ports, lakes, river beaches, heavenly night skies and landscapes wild in Alentejo’s Guadiana Valley National Park, where the river winds its way through steep ravines and kestrels and golden eagles soar above the Pulo do Lobo (or Wolf’s Leap) waterfall.

A detour took us to the eerie ruins of the pyrite mines of Sao Domingos, then we headed to Elvas – a little-known World Heritage-listed border town – just to see the remarkable seven-kilometre Amoreira Aqueduct, which took over 100 years to build (from 1537).

Aqueduct of Amoreira with four floors with small flowers on the grass in the foreground
The 16th century Amoreira Aqueduct in Elvas, Portugal is a Unesco World Heritage Site. Photography: Mauricio Abreu/Alamy

At the start of our journey, we arrived at Vila Real de Santo António, a pleasant border town on the Guadiana estuary with wide seafront promenades and pine forest walks that lead to the dunes of Praia Santo António – the one of the fabulously sandy and often-uncrowded beaches in the eastern Algarve.

Spaniards can ferry across from nearby Ayamonte – to shop (for towels, I’m told), eat fresh tuna, garlic prawns and cod stew at the seafood cafes or sit under the orange trees on Praça Marquês de Pombal. The city’s market square is at the heart of an urban plan devised by the Marquês de Pombal, who oversaw the reconstruction of Lisbon – and then Vila Real – after the devastating earthquakes of the 18th century.

The city’s later prosperity was built on canned fish, an industry that more or less died out in the 1960s, leaving it to be unloved. The redundant canning factories at the Spanish end of town are still derelict, but the center’s ‘pombaline’ architecture has been given a facelift in recent years.

On the square, the Pousada de Vila Real de Santo António, open since July 2021, is a charming base, a set of restored 18th century buildings: a former kindergarten, the headquarters of the local communist party and a part from the neighboring bank, with swimming pools and rooftop terrace overlooking the Guadiana estuary (double from €130).

Painted stripes on a pebble square radiate from a statue
Spaniards come by ferry to visit the Praça Marquês de Pombal in the Algarve. Photograph: Lewis Oliver/Alamy

The next stop was Castro Marim. Just a short drive upriver, this former river port sits between the Guadiana International Bridge (a towering cable-stayed number visible for miles) and a bird-rich marshy nature reserve (flamingos come and go). There’s a medieval castle overlooking the marshes to Spain, but the town is best known for salt – a natural wetland resource that has been mined in the region for eons. Salt craftsman Jorge Raiado offers tours and tastings at his family’s Salmarim salt pans (salt pans) – white with sun-dried crystals that are harvested without machinery or chemicals.

Our phones made erratic changes between Spanish time and Portuguese time as we followed the river on a side road to Alcoutim. The LimiteZero zipline house sits among banks of orange, olive and almond trees and gardens of figs and apricots on one of the most beautiful stretches of the Guadiana. The river beach Praia Fluvial do Pego Fundo is an oasis of cool green water and soft white sand imported from the coast. There is also a castle (first built by the Moors, rebuilt in the 14th century and again 100 years later) and an archaeological museum displaying Roman pottery and a collection of medieval stone board games. Sanlúcar, the city’s Spanish twin, is five minutes away by boat.

Two men in T-shirts and shorts walk on a path above a river lined with buildings
In Sanlúcar del Guadiana, Spain, with Portugal in the background. Photograph: Roger Lee/Alamy

The place is popular with walkers, who come here to start (or end) the Via Algarviana trail, a 300 km footpath that goes from Cap St Vincent to Alcoutim. The 165 km Grande Rota do Guadiana (or GR15) from Vila Real to Mértola also passes through here.

A bright yellow house with a wooden door with wrought iron railings
Vila Real de Santo Antonio was rebuilt after the earthquakes of the 18th century. Photograph: Roger Lee/Alamy

As we drive north, the river heads away from the Spanish border into the Alentejo countryside, meeting Mértola on the edge of the national park. A maze of cobbled streets, cats and crumbling buildings, the small town tumbles down the quays and river piers from the rugged walls of another castle. Since pre-Roman times, it served as a particularly important Gaudiana trading post for Moorish rulers, who transported grain and minerals downriver to Atlantic ports. Beneath the castle walls, the pretty whitewashed church of Nossa Senhora da Anunciação dates from the 12th century and was originally a mosque – one of the few remnants of Portugal’s 500 years of Islamic rule. For five-star views of Mertola from across the river, budget hotel Quinta do Vau offers double rooms from just €40 per room.

Another hour’s drive took us north to Monsaraz, an alluring town on a shale hump rising from the Alentejo plains. mounted – a vast area of ​​holm oaks and cork forests, vineyards, farms and megaliths. Its walled pedestrian streets and whitewashed houses are built of metamorphic flint stone (wear suitable shoes) and offer dreamy views of Portugal’s largest man-made lake, formed by the Alqueva dam.

A winding street between whitewashed buildings with water in the distance
The castle and bell tower of Monsaraz in Alentejo. Photography: Joe Daniel Price/Getty Images

Later, we turned our gaze to the night sky. The Alqueva region is the first in the world to be recognized as a Starlight tourist destination, thanks to clear skies, sparse population, and a collective desire to keep the lights low at night. Set in the forest by Lake Alqueva, the Montimerso Skyscape Country House offers spacious suites and generous terraces with views of the montada and deckchairs for stargazing (from €200 B&B).

We were lucky enough to pick a moonless and cloudless evening for a late-night stargazing session at the “official” Dark Sky Observatory in a former elementary school in Little Cumeada.

In a courtyard, we stood and stared off into space as our guide picked out Pegasus, Taurus, Auriga, and the Milky Way. Andromeda, just 2.5 million light-years away, is, he told us, the farthest we humans can see with the naked eye. We could see it here.

Then it’s down to earth, back as we came. I’d love to do it all again – maybe on the Spanish side of the river – although I’m in no rush to repeat the zipline experience.

The trip was provided by Visit Algarve. For more information see visitalgarve.pt or visitportugal.com