The coastal plain of Rome is depressing and featureless. Fiumicino, the village that gave its name to the airport, has in Bastianello a fine old fish restaurant, but that is about it for concessions to cultural and gastronomic curiosity. Medieval wars depopulated the area and malaria followed, mosquitoes filling a vacuum left by brawling human factions. Malaria, of course, means "bad air" and there was a theory, still not unproven, at least so far as I am concerned, that the disease's source was the poisonous breath of serpents swept up from Africa and deposited here by the scirocco. It's that sort of place.

The coastal plain of Rome is depressing and featureless. Fiumicino, the village that gave its name to the airport, has in Bastianello a fine old fish restaurant, but that is about it for concessions to cultural and gastronomic curiosity. Medieval wars depopulated the area and malaria followed, mosquitoes filling a vacuum left by brawling human factions. Malaria, of course, means "bad air" and there was a theory, still not unproven, at least so far as I am concerned, that the disease's source was the poisonous breath of serpents swept up from Africa and deposited here by the scirocco. It's that sort of place.

There is certainly something spooky and malignant about the Spiaggia Romana, Rome-on-Sea. Places abandoned by the Romans are always haunted. Even today, surrounded by air-conditioned Day-Glo Kassböhrer coaches, Pompeii remains absolutely terrifying. There is a tangible horror in the air - bad things happen on this coast. It was on the beach near Ostia that poet and film-maker Pier Paolo Pasolini paid the high price of cultivating very rough trade in 1975. He had predicted his death just days before, citing an involvement with the Mafia and prostitution.

But there are many other ghosts too, ancient and modern. Tiberius and Marcus Aurelius had their seaside villas here. Later, at his holiday house in Fregene, Federico Fellini entertained in the decadent style of La Dolce Vita: weekends out from working at Cinecitta, they replicated Imperial Roman debauches with the benefit of 20th-century facilities. Fellini once said he could only make love if wearing a bra, but other more archaeological aspects of complexity and meaning fascinated him too. He was acutely aware of the layering of cultures. Describing his 1972 movie Roma, he said: "I wanted to get across the idea that underneath Rome today is ancient Rome. I am conscious of that and it thrills me. Imagine being in a traffic jam at the Coliseum! Rome is the most wonderful stage set in the world."

It is the old Roman via Aurelia that connects the capital with the coast, although there is no sense of ancient romance on the surface. The Mercedes taking us to Palo Laziale and one of the strangest, most marvellous, most layered hotels in the world has an execrable soft rock station playing at only just above the threshold of hearing, as irritating as the tinny thrash of a fellow passenger's Walkman.

Not all of Italy is romantic. We pass a garage called Alfa Queen, a manufacturer of concrete pipes, a factory outlet for sports goods, an electrical component distributor and, as we approach Ladispoli, our destination just 30 clicks from Rome, the faded and abandoned detritus of a circus rusting behind chicken wire...as fitting a memorial to the ghost of Fellini as you could imagine. A stage set, indeed.

And then we slip off the road, out of the 21st century and into a wood. Not exactly Dante's " selva oscura", but mad axeman territory nonetheless: furtive coves in tracksuits loiter by trees, the way they often do in Italy. Later we learn that they are, at least according to local prejudice, Poles and Albanians. We then encounter a vast wooden security gate with a microphone entry system. Following commands, it anonymously slides away to reveal a curving drive through more woods which eventually delivers us to a formal garden (with African trees perhaps borne here by the same scirocco that deposited those foul-breathed serpents). There is a single, Dutch-registered Ferrari 550, and beyond that an ochre palazzo of striking presence and dignity. When the Mercedes' engine is switched off there is absolute silence. No traffic. Not even birdsong. Just the sound of the Tyrrhenian Sea washing up on the black sand on the other side of the building.

This is La Posta Vecchia, a hotel since 1990 and an establishment as deposited with complex meanings as Fellini's Rome. Thirty years before, it had been bought by Jean Paul Getty, at the time the richest man in the world and author of some terrific one-liners including: "the meek shall inherit the earth, but not its mineral rights". Before Getty it had been a post house, a place for coaches to stage while taking the mail to Rome. It was built in 1640 by the Principe Odescalchi, whose much older castello is next door - a dramatic visual contrast in its monumental masonry. You do not have to be an art historian to notice that the two buildings, one orange, the other grey, each strapped to the shining sea, look like a painting by Claude Lorrain, the Alsatian paysagiste who made idealised visions of Roman landscape his signature and fed them into our imaginations, so that every English park is a version of a Frenchman's take on the Roman campagna.

Just after the First World War the old post house fell into disuse and became a ruin until Getty persuaded the Odescalchi of the day to sell it in 1960. We can only speculate about the negotiations but it cannot have been entirely straightforward: old and new money rarely make good neighbours, and while the castello and the posta are completely adjacent buildings, vast chasms of architectural history and psychological space separate them. The academic resources of The Getty Museum were used for a meticulous restoration of La Posta Vecchia. Beneath the late Renaissance building, the work revealed the ruins of a couple of villas from the second century before Mel Gibson. Our luxury hotel was layered on the remains of the Roman town of Alsium, last mentioned in 547AD and soon after in the very steepest decline. But the ghosts never leave and they insist on continuing to have their say: late in life the already cantankerous Getty, perhaps demented by the isolation here, became convinced he was a reincarnation of the Emperor Hadrian.

La Posta Vecchia has nothing of the conventional luxury hotel. No formal reception, no bell-hops, no shops, no carpet, no bar, no themed restaurants. There is a lift, but it is hidden. Instead there is an exceedingly rich man's house, more or less as he left it, with the minimum of concessions to the mechanics and graphics of the contemporary hotel trade. "Public" rooms on both floors are all disposed along a spine and run into one another, at different levels. The floors are stone. The unsettlingly huge size and scale of the spaces is compensated by the steadying and solemn gravity of vast and impressive antiques of a profoundly masculine character. A fountain plays, the light comes off the sea.

Severe stone steps lead upstairs to the rooms. And then there is more of the silence which is absolute and deafening, except, again, for the wash of a sea that Odysseus knew. The bedrooms have Palladian proportions and the bathrooms are imposing: marble of imperial dimensions, swan taps in brass with just enough verdigris to confer distinction and character, not a sense of neglect. There is cavernous vastness; Bulgari ointments; a Tiberian supply of white towels. Amusingly, our bathroom had a money plant. There is a heavy presence of calm and isolation: big bed, big velour sofas, real paintings, Chinese things, an armoire that might have been used by the Inquisition to store dead bodies. The sheets are fine linen by Pedersoli. The bedrooms have no balconies, but those on the west have sublime sea views. You stand and stare and listen to nothing, and, just as you are grateful there is no fruit basket, the kitchen sends up a welcome drink. From the whizzing centrifuga comes a strange frog-green smoothie made from apple, banana and ginger. None of these, you ruefully note, is an indiginous Italian ingredient.

La Posta Vecchia has a classical dining room where muted Vivaldi CDs play. One sunny Thursday lunchtime in April it had just two magnificent mottled old boys as guests, one a dead ringer for Enzo Ferrari with his dark glasses, cornflower blue shirt and pastel yellow cardigan underneath a blazer, drinking and eating with fine slowness and deliberation. Nearing the ends of their lives, there was no need for this pair to rush lunch, of all things.

But we wanted the sun and went, unseasonably, to the terrace. A bottle of oil is immediately set down: Le Lupinate from Montiano in the nearby province of Grossetto. It is fine and yellow and peppery. Good bread, an anticipatory glass of prosecco. Absolute perfection...until the main course. The cook's curriculum vitae, not to mention the exotic products of his centrifuga, hint at some ambition, not to say pretension. Michele Gioia has worked at Alain Ducasses' Spoon in London's Sanderson Hotel (in my view by a clear margin the worst smart restaurant in the world), although more promisingly he has done time at The Four Seasons in Milan. Somewhere he picked up an idea that his visitors do not want Italian food, as odd a conceit as Ferrari deciding to make panel vans. The menu has that annoying complexity which punishes Italians when they stray from their essential competence in natural simplicity. Eating by the sun and the sea I do not want, but am given, an amuse-bouche of spatchcocked prawn on a purée of Jerusalem artichoke with highlights of beetroot foam. Larger dishes, again, rely on an unsettling and unnecessary cosmopolitan complexity: there is a mille foglie di salmone fondente con crema verde e salsa allo zenzero (a mille-feuille of salmon with green ginger sauce) which is, frankly, a little bit of nonsense. They do not, nor have ever had, salmon in Rome: it is as silly as eating swordfish in Glasgow. They do pasta fatta in casa; the Garganelli integrali con carciofi a ragu leggero d'anatra (wholemeal garanelli with artichoke and a light duck sauce) was disappointing...and there was not much of it. The taste of wholemeal flour was too apparent, and the duck ragu, so far from giving the gaminess the dish needed, was so light it was undetectable. A spaghetti cacio e pepe e olive taggiasche (spaghetti with cacio cheese, black pepper and olives) did not use the advertised olives. Hmmmm: says something about the visitors they expect. The service was, however, impeccable and elegant. There are no great regional wines in Latium, but a bottle of the Casale del Giglio unoaked chardonnay was delicious. It was just such a pity that it did not accompany an honest, generous and robust dish of clams and pasta.

The next day at breakfast a fruit plate had pineapple, kiwi, strawberries and grapes in another unfortunate departure from Italy.

There is a small indoor pool, but La Posta Vecchia would not be an ideal centre for triathlon training: those layers of archaeology have mocked modern hedonism and frustrated excavations for an outdooor version. La Posta Vecchia's intense charm is not in its hotel facilities, but its essential character. They will lend you a mountain bike, but there is nowhere to go. I asked the ebullient manageress about Ladispoli and she wrinkled her face in a caricature of horror and disgust, hinted at the negative effects of immigration from eastern Europe and suggested instead a short ride to the Marina di San Nicola, about a mile away. Here again was a bracing corrective to any sentimentally inclined view of Italy as an earthly paradise. When you say to me "Marina di San Nicola", a vision swims into view of stucco houses and faded blue shutters; bright water; wooden fishing boats; little bars and tratts serving fish so fresh it wriggles; beautiful women on scooters. And that's where I am wrong. Actually no. Marina di San Nicola is a low-rise housing estate whose only point of interest is a brightly lit tent where you can play video games while listening to country and western music. The dark heart of Italy is, I am afraid, really rather garish.

But it does afford you a wonderful view of La Posta Vecchia which, from a municipal car park, looks even more like something dislocated from history: its position and mood are spectacularly picturesque. These are its pleasures, although it is as well to recognise that the reasons J Paul Getty came here are, perhaps, not the same reasons you would want to. His preoccupation with security was so profound that guests have no access to a beach that is now sealed off. What, I found myself wondering, were Getty's mornings like, left here in extraordinary beauty and isolation? You would, I suppose, become a polymath. Or this is what I thought to myself as I watched my wife reading Edith Wharton at ten in the morning.

You would certainly start excavating, as he did. Many fine hotels have history, but La Posta Vecchia's is surely the densest and most archaic: underneath it is a genuine museum full of amphorae and lanterns. The Opus Reticulatum masonry remains, as do beautiful mosaic floors. There is a magnificent barrel-vaulted cistern that acts as a natural fridge for storing oil and wine.

In the corridor outside our bedroom was a monumental Venetian mirror, its glass totally occluded, so that you could imagine a ghostly reflection of Casanova. La Posta Vecchia lends itself to this sort of poetic speculation. For conducting a secret affair there can be nowhere better: Getty's neurotic need for isolation serves the modern philanderer as well as it did the billionaire oilman. And when you visit the Getty suite itself, these thoughts redouble. It is dark, heavy and powerful in green and brown with drawn shades. What weird trick of history brought American oil here? The Getty suite has a throne! Next door is the Medici suite, reserved for his mistresses: he left La Posta Vecchia to four of them on his death. Poetically, an enormous capriccio of ruins hangs in a gilt frame, mocking human vanity and frailty. The mistresses had access to a contiguous bathroom accessed by an extravagant marble stairway that would flatter most ballrooms. Indeed, the stairway would flatter most women.

At night you go to Rome to eat, just 40 minutes away in the Mercedes that La Posta Vecchia summons from nowhere for you. Eat somewhere modest, say, La Buca di Ripetta in the Tridente, the network of three big shopping and antique streets fanning out from the Piazza del Popolo. Here you get a fritto misto vegetariano, octopus and rocket and traditional Roman saltimbocca. And there is da Giggi, rough and delicious: a frittata of artichokes and oxtail with local merlot. Or cross the river and go to Trastevere: the night we were at the austere, but wonderful, La Tana de Noantri, so was The Doncaster Virgins' Club (Monolingual Section) - six chubby, jolly women from Yorkshire, all loudly wondering if they got anything with the lamb chop. We ate fried fish and tried to look local. And just as this coarse Roman cooking is a relief after the lofty artifice of La Posta Vecchia's cucina, so it was on every night a delicious experience to return from busy Rome to the sensational calm and fundamental beauty of Getty's old house.

You get a sense at La Posta Vecchia that there are guests of Getty who have left or, for some reason, just cannot be here at the moment but may arrive at some point in future. Watching the moon over the inky sea and a distant jet on the glide-path you imagine the grumbling of the old man, the squeals of mistresses...even the echoing laughter of the Romans. And that Venetian mirror? Edmund Wilson said no one ever bettered Casanova in his account of "the passing glory of the personal life". Ah yes...

La Posta Vecchia makes you think these things. It is as romantic and as layered and as hauntingly beautiful as Rome itself. It is, as Fellini, said just like a stage set. It is one of the most remarkable hotels in the world.

TRAVELLER'S GUIDE

GETTING THERE

Rome's Fiumicino airport is the closest to the hotel. British Airways (0870 850 9850, www.ba.com) flies there from Birmingham, Manchester, Gatwick and Heathrow. Alitalia (0870 544 8259, www.alitalia.co.uk) flies from Heathrow, and Globespan (0870 556 1522, www.flyglobespan.com) from Glasgow (and, as of next month, from Edinburgh).

La Posta Vecchia is about 40km from Fiumicino airport. The hotel offers a limousine transfer costing€98 (£70) one-way. There are no direct public transport links with the hotel.

STAYING THERE

La Posta Vecchia (00 39 06 994 9501, www.lapostavecchia.com) offers double rooms from €570 (£407) per night including continental breakfast.

You can get lower prices through a tour operator. The writer travelled with Exclusive Italy (020-8256 0231, www.exclusiveitaly.co.uk), which offers three nights at La Posta Vecchia from £879 per person, based on two sharing. The price includes accommodation with breakfast, return scheduled flights from Heathrow or Gatwick to Rome, and car rental. This price is valid for travel until 7 November 2004.

In Rome, the Bed and Breakfast Association of Rome (00 39 06 5530 2248; www.b-b.rm.it) has more than 100 B&Bs and apartments to choose from around the city. A budget option is Colors Hostel (00 39 06 687 4030; www.colorshotel.com) at 31 via Boezio. Double rooms cost from €75 (£54) and dorm beds €22 (£16) per night.

EATING THERE

In Rome, the writer ate at La Buca di Ripetta (00 39 06 321 9391) at 36 via di Ripetta. Also, La Tana di Noantri (00 39 06 580 6404) at via della Paglia, and Trattoria di Giggi (00 39 06 679 1130) at via Belsina. No visit to Rome is complete without an ice cream; head to Tre Scalini (00 39 06 6880 1996) in Piazza Navona for its famous tartufo. Giolitti (00 39 06 699 1243), at 40 via Uffici del Vicario, is the oldest institution.

FURTHER INFORMATION

Romaturismo (00 39 06 488 991; www.romaturismo.com) is located at Via Parigi 5 and is open Monday-Saturday from 9am-7pm.

The Italian State Tourist Board (ENIT) (020-7399 3562; www.enit.it).