Strapped into a harness, hands clamped to a metal bar, I flew – at terrifying speed along 720 metres of zipwire suspended over the Rio Guadiana – from Spain into Portugal. I’d taken a boat from Alcoutim (on the Algarve side of the river) to Sanlúcar de Guadiana (in Andalucía). From there I was driven up to a launch platform on a rocky summit with jaw-dropping views of both countries. Did I scream? As I hurtled towards Portugal, I tried to focus on those views: two dazzling white villages, the wide green river below, a castelo on one side, a castillo on the other. It was all over in less than a minute, but thanks to the international time difference I gained an hour. The LimiteZero experience (the world’s only cross-border zipline) was just one of the high points of a meandering road trip along the Rio Guadiana – the long river that rises in the Spanish province of Albacete, slips over the Portuguese border near Elvas in Alentejo and heads south to the Bay of Cádiz, on the Algarve’s eastern edge. Driving a hired Fiat 500, my husband Dave and I stuck mostly to 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 route, the river forms a natural boundary between Spain and Portugal; a trail of castles and fortresses glare at each other from opposite riverbanks. Too many castles for one trip, perhaps, but there are lots of other things to see: salt marshes, walled towns and ancient river ports, lakes, river beaches, heavenly night skies and the wild landscapes of Alentejo’s Guadiana Valley national park, where the river squeezes into craggy ravines and kestrels and golden eagles circle over the waterfall at Pulo do Lobo (or Wolf’s Leap). A detour took us to the eerie ruins of the Sao Domingos pyrite mines, and later we pressed on to Elvas – a little-known border town with world heritage status – just to see the remarkable seven-kilometre Amoreira Aqueduct, which took more than 100 years to build (from 1537). At the start of our trip we rolled into Vila Real de Santo António, a pleasant border town on the Guadiana estuary with wide marina promenades and pine-forest walks that lead to the dunes of Praia Santo António – one of the fabulously sandy and often-uncrowded beaches of the eastern Algarve. The Spanish can nip across on a ferry from nearby Ayamonte – to shop (for towels, I am told), eat fresh tuna, garlic prawns and codfish stew at seafood cafes or sit under orange trees on Praça Marquês de Pombal. The town’s market square is at the heart of a masterplan dreamed up by the Marquês de Pombal, who oversaw the reconstruction of Lisbon – and then Vila Real – after devastating earthquakes in the 18th century. The town’s later prosperity was built on tinned fish, an industry that more or less died in the 1960s, leaving it to look unloved. Redundant canning factories at the Spanish end of town still lie derelict, but the centre’s “Pombaline” architecture has been spruced up in recent years. On the square, the Pousada de Vila Real de Santo António, open since July 2021, is a delightful base, a collection of restored 18th-century buildings: a former kindergarten, the headquarters of the local Communist party and part of the bank next door, with pools and roof-terrace views of the Guadiana estuary (doubles from €130). Next stop was Castro Marim. Just a few minutes’ drive upriver, this ancient river port sits between the Guadiana International Bridge (a towering, cable-stayed number visible for miles) and a marshy nature reserve rich with birdlife (flamingos come and go). There’s a medieval castle with views across the marshes to Spain, but the town is best known for salt – a natural wetland resource which has been mined in the region for aeons. Artisan salt panner Jorge Raiado offers tours and tastings at his family’s Salmarim salinas (salt pans) – white with sun-dried crystals which are harvested without machinery or chemicals. Our phones made erratic switches between Spanish and Portuguese time as we followed the river on a back road to Alcoutim. The home of the LimiteZero zipline sits among banks of orange, olive and almond groves and gardens of figs and apricots on one of the loveliest stretches of the Guadiana. The Praia Fluvial do Pego Fundo river beach is an oasis of cool green water and soft white sand imported from the coast. There’s also a castle (first built by the Moors, reconstructed 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 town’s Spanish twin, is five minutes away by boat. The place is popular with walkers, who come here to start (or finish) the Via Algarviana trail, a 300km footpath that runs from Cape St Vincent to Alcoutim. The 165km Grande Rota do Guadiana (or GR15) from Vila Real to Mértola also passes through. As we drive north, the river swings away from the Spanish border into Alentejo countryside, meeting Mértola on the fringe of the national park. A labyrinth of cobbled streets, cats and crumbling buildings, the little town tumbles down to river wharfs and jetties from the rugged walls of yet another castle. Since pre-Roman times, it has served as a Gaudiana trading post, particularly important to the Moorish rulers, who shipped grains and minerals downriver to the 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 relics of Portugal’s 500-year Islamic rule. For five-star views of Mertola from the opposite side of the river, budget hotel Quinta do Vau has doubles from €40 room-only. Another hour’s drive took us north to Monsaraz, a beguiling town on a hump of schist that rises from the plains of Alentejo’s montada – a vast area of holm oaks and cork forests, vineyards, farms and megaliths. Its walled, pedestrian streets and whitewashed houses are built of flinty metamorphic rock (wear sensible shoes) and offer dreamy views of the largest artificial lake in Portugal, formed by the Alqueva Dam. Later we turned our gaze to the night sky. The Alqueva region is the first in the world to be recognised as a Starlight Tourism Destination, thanks to clear skies, a sparse population and a collective drive to keep the lights low at night. Set in forest on the edge of the Alqueva lake, Montimerso Skyscape Country House has roomy suites and generous terraces with views of the montada and deck chairs for stargazing (from €200 B&B). We were lucky to choose a moonless, cloudless evening for a late-night stargazing session at the “official” Dark Sky observatory, in a former primary school in tiny Cumeada. In a courtyard we stood and stared into space while our guide picked out Pegasus, Taurus, Auriga and the Milky Way. Andromeda, a mere 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 the way we came. I’d like to do it all again – maybe on the Spanish side of the river – though I am in no hurry to repeat the zipline experience.