The series starred Patrick McGoohan, a rising young actor with writing and producing ambitions. He'd made his mark as John Drake in a superior TV drama called Danger Man that appeared around the same time as the Sean Connery Bond movies.

But The Prisoner was in a different league. McGoohan played "Number Six", a former British spy who had been mysteriously condemned to wander through a people-less, exquisitely picturesque resort town that was predominantly white-walled and surreal.

The real life location for Number Six's prison was the North Welsh resort of Portmeirion. But today I'm experiencing my own Number Six moments in Monsaraz, on Portugal's border with Spain.

Like Portmeirion, Monsaraz is white-walled, surreal and people-less (at least on this day).

It's winter, which partly explains the lack of humans on the cobbled streets. But there's an odd feeling about the place, as if we have wandered into a deserted movie set.

Monsaraz is certainly achingly beautiful, occupying a commanding position on a steep hill with commanding views above the Guadiana river, which forms the border in this part of the Iberian peninsula. These days the town is a glorious backwater, its narrow medieval streets completely free of cars, making it a delight to stroll around. Once, its imposing walls provided refuge from attackers, but today the younger generations have moved to the surrounding plains where life is more convenient.

In its heyday, Monsaraz was an important strategic fortress, the scene of many a major battle between the Christians and the Moors, or the Portuguese and the Spanish.

For a period after 1167, when it was recaptured by Geraldo Sem-Pavor (early Portuguese for "The Fearless"), Monsaraz was a stronghold of the powerful Knights Templar.

Then, of course, the all-too-powerful order was disbanded by Pope Clement IV in 1312.

Leap forward 70 years. In 1381, the town came under attack from an unexpected foe. The English troops of the Earl of Cambridge were meant to be allies of the Portuguese king, Ferdinando I. But you know how soldiers get when they haven't been paid and they are angered by a foreigner's snub (particularly when they happen to be in the foreigner's land). What was Ferdinando I thinking? It's one thing not to pay your foreign mercenaries, but when you also renege on your deal to marry off your daughter to the aristocratic Englishman who has brought the English troops out here, what did he expect? So his troops ransacked Monsaraz in retribution.

We're part of a more peaceful invasion. We leave our Insight Vacations' coach at the car park outside the ancient walls and enter Monsaraz – as most visitors do – through the Porta da Vila, the massive gate that leads to the main street, Rua Direita.

The houses are typical of the Alentejo region: whitewashed, red-roofed, decorated with wrought-iron balconies and hanging baskets of flowers, and with distinctive, elongated chimney pots.

The obvious destination is the town's formidable castle. But there's no hurry, so each of us strolls around at our own pace, taking photographs, popping into the few shops that are open at this time of year, and generally savouring the peacefulness of the place.

I take a few moments to enter the 16th-century church, Igreja Matriz, and admire its gilded altars and painted pillars. Across the street, the former law court is now the Museu de Arte Sacra, with a fine collection of religious books, vestments and sculpture.

On this morning, none of the town's restaurants – specialising in local dishes such as lombo de assado no forno (pork) and rego assado (lamb) – are open. But we're assured Monsaraz isn't always as quiet as this. On the last Sunday of every month the town hosts a popular flea market, while September and Easter witness the bull-fighting season.

Eventually we all gravitate to the castle. Originally built by Alfonso III and Dinis in the 13th century, it was substantially reinforced in the 17th century. The views from the keep are spectacular in every direction.

This is one of Portugal's designated wine regions and we can see the vineyards stretching out before us. Monsaraz is also surrounded by some of the amazing megaliths famous in this part of Portugal. These massive and mysteriously hewn stones are believed to be have had some fertility purpose for the people who lived here up to 6000 years ago.

But if those megaliths haven't changed in generations, the view from the castle over the Rio Guadiana has.

In 2002, the Guadiana was dammed, creating the Alqueva reservoir, Europe's largest artificial lake at some 83 kilometres long.

We leave Monsaraz in our coach and 10 minutes later – having spotted a Knights Templar guard post, a couple of megaliths and several Iberian black pigs en route – we arrive at the reservoir to board our mid-morning cruise.

Let's be honest. There's nothing particularly authentic about this experience. The lake is artificial. On its shores, we board the Westlander, imported overland from the Netherlands where it began life as a live-aboard, shallow-bottomed, twin-keeled cargo boat in the early part of the 20th century. Even Tiago Kalisvaart, our host, is more Dutch (or Spanish) than Portuguese.

But having said that, we enjoy a magical couple of hours cruising, sometimes under sail, over this gigantic inland waterway that straddles the border of two great seafaring nations.

Alqueva reservoir remains a controversial subject in both countries.

The concept was first proposed in the 1950s by Portugal's long-serving fascist dictator, Antonio Salazar, as a means of providing water to the country's driest and poorest region, Alentejo. But construction didn't start until 1998, and only then because Portugal decided to press ahead without Spanish sign-off – quite an issue, given the lake has spread out over the two countries.

Several villages and wildlife habitats were drowned in the process. Some consider the reservoir to be an environmental disaster. Others say it is a pointless exercise, that the Alentejo will always be both hot and poor, with a constant exodus of the young to wealthier cities, in Portugal but usually much further afield.

But here we are, enjoying canapes of local cheese, chorizo, olives and delicious home-made bread washed down with Alentejo vino, and the lovely lakeside views from the Westlander.

Unfortunately, we can't linger long enough to show remorse. Tiago's family also owns a restaurant-cum-art gallery in the nearby town of Telheiro.

In a former life, Restaurant Sem-fim, in Rua das Flores, was an olive-oil mill. Once it closed down, it was bought by Tiago's Dutch-born father, a sculptor. Tiago's Spanish-born mother was equally involved in the family business, making the rugs for sale at the restaurant and writing (what Tiago ensures us) is the definitive guide book to the Alentejo.

And Tiago has expanded it, recently launching another tourist business. Three times a week, on Tuesdays, Fridays and Saturdays, the restaurant hosts star-gazing sessions with telescope and sky guide. Dark Sky Alqueva celebrates the fact that this underpopulated corner of Europe is one of the finest places in the world to witness spectacular glimpses of the celestial.

Tiago tells us that roughly 50 per cent of all nights in Alentejo are cloud-free (probably why it is so dry and poor).

I would have loved to have experienced the dark sky excursion, especially coming from an Australian city that has among the most light-polluted night skies on the planet.

But we suddenly have to dash. Apparently the Moors might be on the move again.

​TRIP NOTES

MORE INFORMATION

visitportugal.com/en/destinos/alentejo

GETTING THERE

Emirates Airline has three daily flights from Sydney and Melbourne via Dubai to Lisbon and Madrid. See emirates.com/au.

TOURING THERE

Insight Vacations' 10-day Country Roads of Portugal visits Lisbon, Portimao, Evora, Tomar, Douro Valley and Porto, and includes a visit to the Cathedral in Evora. Priced from $2625 per person, twin share (single supplement from $780).  See insightvacations.com or phone 1300 301 672.

The writer was a guest of Insight Vacations.