There is a view from the monastery of Serra do Pilar that Arthur Wellesley, 1st Duke of Wellington, may have enjoyed on the eve of his assault on Porto, during the Peninsular War.

Gazing across the Douro river from Vila Nova de Gaia, he would have surveyed the medieval row of merchants’ houses lining the bank and the steep hill behind them, crowned by a formidable cathedral-fortress. Only the iron Dom Luis I bridge, built in 1886, would have been missing from the duke’s view.

As the sun sets, the scene is illuminated by a glow of bulbs, reflected in the river.

Porto by night Credit: AP

The city of Porto, built of granite in an eclectic mix of picturesque medieval, flamboyant baroque and grandiose neoclassical architectural styles, is the embarkation point for cruises along the Douro.

This is the river on which, until the mid-Sixties, square-sailed barcos rabelos – a throwback to ancient Phoenician trading vessels – braved rapids and other hazards to the ferry port from wineries upstream.

They ended up – as did we, on our city tour – in the cellars of Gaia, whose port lodges, with names like Dow’s, Graham’s, and Taylor’s, are testament to the 200-year-old presence of British commercial interests in these parts.

Porto Cathedral Credit: AP

My ship, however, was not British but French, with mostly French-speaking passengers. The MS Gil Eanes is one of the newest ships in the CroisiEurope fleet, a comfortable vessel whose no-frills approach is reflected in the cruise fare. Her 66 cabins, decorated in crisp white and turquoise, pack in generous storage and a TV that folds into the ceiling.

Lighting is strong enough for failing middle-aged eyesight to read by, showers have oomph and, best of all, the floor-to-ceiling windows allow you to experience the sights and sounds of dawn from the comfort of bed.

Little beats the waking view of an eagle, swooping down to soar from the water, breakfast in his talons.

I spent cruising hours on the sun deck, enjoying spectacular views throughout our 125-mile journey. The river’s most treacherous rocks were blasted away in the 18th century, and dams and locks now allow boats a gentle passage as the Douro meanders through valleys of schist, narrows through granite gorges and widens again into an arid, fig-fragrant landscape towards the Spanish border.

"I spent hours on the sun deck, enjoying spectacular views" Credit: AP

Vineyards were the leitmotif of our trip, cascading down hillsides on terraces bolstered by traditional dry stone walls, known as socalcos. In the world’s oldest demarcated wine region (producing since 1756), vines form kaleidoscopic patterns of green as evolving viticulture techniques exploit the undulating land.

We saw elegant quintas, the odd chapel on a hilltop, an iron bridge over a tributary. The silence is palpable in the fierce summer sun, the only sounds the twitter of birds, ratchety crickets and the ripple of lamprey in the water.

The Douro was not mapped until 1848 – by the English scion of a port family, James Forrester. He was ennobled for his work on the river that was to claim his life, in 1861. We passed the spot at Valeira gorge where his boat capsized after dinner with the visionary doyenne of Quinta do Vesúvio, Dona Antónia Ferreira. She survived, buoyed by her crinoline; Forrester, weighed down by gold coins, drowned.

I would have welcomed a guide on board to share knowledge of flora and fauna or nearby towns, but local guides were available only for optional shore tours (with French and English commentary).

The undulating vineyards of the Douro Valley Credit: Getty

One excursion took us to Braga, 30 miles north of Porto. Portugal’s ecclesiastical capital is famed for its churches and magnificent cathedral, the pipes of its ornate twin organs radiating like a blazing sun from the high choir.

Upstream, we visited the Palace of Mateus, with its two-tiered gardens, and the town of Lamego, home of olive oils, honeys, Iberian ham and sparkling Raposeira wine, sold in narrow streets in the shadow of the Sanctuary of Our Lady of Remedies. Pilgrims, it is said, climb the 686 steps to the baroque church on their knees, pausing on the terraced stairway to admire the religious tableaux of blue and white azulejo tiles.

Braga, Portugal's ecclesiastical capital Credit: AP

Except for lunch on our day in Salamanca – a long drive across the plains to a magnificent city better visited at leisure – we ate on board. With dishes such as magret de canard au porto et à la figue and filet de daurade en “caldeirada”, efforts to tickle our palates generally succeeded.

Mealtimes should be convivial occasions, and it’s likely you’ll find common ground with at least some fellow passengers. But the single-service fixed-seating system means that, with 19 meals in the company of the same five people, a lot rides on the seating lottery.

My favourite excursion, the Wine Route, followed a historical thread, culminating in the stylishly modern Quinta do Seixo winery. We wound up the N222, voted “most beautiful road in the world”, to São Salvador do Mundo, for a raptor’s-eye view, at 1,617ft, over the river. It was a short hop to São João da Pesqueira, traditional centre of wine production.

Deserted during siesta hour, its cobbled square, surrounded by whitewashed and carved-granite baroque buildings, resembled an opera set for a duet between King José I and the Marquis of Távora. Távora was executed in 1759 for treason, his lands confiscated. But his family’s imposing mansion and chapel are a reminder of the old nobility’s role in the 2,000-year history of the Douro vineyards, where today 36,000 growers make a living. Offering a glimpse of local life, it left me wanting more.

Essentials

A seven-night Douro river cruise on the MS Gil Eanes with Orbital Travel costs from £1,459pp, departing on July 26, 2018. Includes flights (01763 274174; orbitaltravel.co.uk).