The other side of the story

'The city, his city, stood unchanging on the edge of time: the same burning dry city of his nocturnal terrors and the solitary pleasures of puberty, where flowers rusted and salt corroded, where nothing had happened for four centuries except a slow ageing among withered laurels ..."

So Gabriel García Márquez describes Cartagena, for much of his life his own adopted city, in his novel Love in the Time of Cholera. The description largely stands the test of time, just as the city itself. The 16th-century colonial town still perches, walled and turreted, on the Caribbean shore. The bougainvillea tumbling from the balconies in the narrow streets still "rusts", the salt still corrodes, the air is still full of solitary (and not so solitary) pleasures and the less frequented streets may still be witness to the odd nocturnal terror, although many fewer than in other Colombian cities, and certainly no more than streets in London or Manchester.

But nothing happening? That's certainly no longer the case. An awful lot happens in Cartagena: international film festivals, regattas, bull-fighting seasons, a national beauty pageant and from now, once a year, those withered laurels will be dusted off for the Hay Festival of Literature.

Love in the Time of Cholera is the perfect companion for a visit to Cartagena, and you could easily navigate the city by following Márquez's fiction rather than a conventional guide book. The novel is just as useful a barometer and guide for the more abstract aspects of the city, too, for negotiating its spirit, its soul. Ironically, however, you might find that your experience of Cartagena will somewhat diminish the imaginative power of Márquez's writing, simply because in Cartagena so much apparent invention lives before your eyes.

Although Márquez now lives mostly in Mexico, for the duration of the festival the city and the novelist share some actual territory once again. On the second day of the festival, I'm lucky enough to spot the man himself as he sits down in front of a giant plasma screen in the old chapel of the elegant Charleston hotel to watch Vikram Seth speak at an event held just down the road in the Claustro de Santo Domingo. I nervously approach him with my copy of Love in the Time of Cholera. He takes the book, notices the second-hand pencilled "£3", raises an amused eyebrow and carefully signs, "Para Owen, su amigo, Gabo."

I'm touched. But then I remember that in Cartagena Gabo is everyone's friend, or rather everyone is Gabo's friend. The whole city seems to either reflect or be in dialogue with his writing. Sometimes it can be hard to work out which way the influence flows. Is the talk of love by the drinkers reclining on cushions on the city's walls informed by the fact that Gabo's books are on every school syllabus, or is their attraction to the big abstract nouns of love, grief, anger, passion, a genuinely Colombian trait?

A bit of both I suspect, and it seems I'm not immune to it either. For my whole time in Cartagena I can't help seeing the close juxtaposition of the physical and the abstract, the body and the spirit that constitutes so much of Márquez's magic realism, echoed in the city about me again and again. On the road up to the Convento de la Popa, the Catholic festival of the Virgin of the Candelaria (the patroness of the city) is advertised in an Aguila beer tent next to a massive poster of the pneumatic and bikinied Aguila girls; outside the city walls, a saddled horse stands patiently tethered between some goalposts on a deserted football pitch; boys sell time on street corners, their hands full of mobiles, placards about their necks advertising "300 pesos for 5 minutes, 500 pesos for 10"; the narrow streets are suddenly filled with a procession of young people wearing gigantic letters of the alphabet; at two in the morning, a painting apparently floats across an empty piazza, until we see the bare-footed legs beneath. I could continue, but I think I've made my point.

There is one area in which ideas of the body obviously dominate over ideas of the spirit, and that is in a certain class of Colombian's fondness for surgical enhancement. Although several men I spoke to wished to believe the contrary, a vast number of Colombian breasts seem to be, well, a bit "magically real". Combine this with the fact that the size of T-shirt worn is nearly always in inverse proportion to the size of enhancement, and you can't help wondering what much of the female population are going to look like in 50 years when the rest of their bodies have succumbed to the ravages of time and just their cleavages are left stubbornly defying gravity. If Márquez were writing his novel now, Love in the Time of Silicon might be an apt title.

Of course dramatic enhancement is very much in Cartagena's history, as is evidenced by Las Murallas, the 8km of impressive walled defences that almost, but no longer quite, encircle the old town. These were constructed over two centuries to defend the city against marauding pirates.

The attraction for these pirates lay in the huge stores of looted native treasures that Cartagena held while waiting for the twice-yearly visits of the Spanish galleons. In the 16th century alone, the city endured five pirate sieges. In 1741, a massive English sea assault by Edward Vernon, second in size only to the D-Day landings, also failed to break the defences. Combine this with Cartagena's early declaration of independence in the 19th century, and you can see why Simón Bolívar named the city La Heroica.

These days the walls play host to young lovers and shots of a different kind, served from a bar not a cannon. The role of the walls in a modern Colombia also seems to be somewhat reversed, in that nowadays they are as much about keeping people (foreign tourists) in, as keeping others out.

More than once I was advised not to wander beyond the walls on my own. This made me suspect the old town was something of a theme park, which, given its perfect renovation after achieving Unesco world heritage status, you could argue it is. But only in El Centro. Take a stroll through some of the further winding streets and you're left in no doubt that the old town is still very much a living, breathing, working society. Wanders outside of the walls are also fine, through the stunning "Republican" and Moorish architecture of La Manga, and around the harbour and down the long Miami-like strip of the Bocagrande. Any further, however, out towards the sprawling barrios, and you might find yourself in trouble. I'd hoped to visit Nelson Mandela barrio, an entire community of displaced people from the interior of the country. The Hay Festival together with Plan International ran a kids' poetry project here. I was intrigued but it was made clear to me that without prior planning and security, Nelson Mandela was strictly out of bounds.

Colombia's internal troubles are hardly a well-kept secret. It's well known that Spanish America's oldest democracy is still the most dangerous place in the world to be a trade union leader, and when you plan to visit, trouble is all you hear about. The Foreign Office site will warn you off most of the country, and your friends will tell you you'll be kidnapped. As ever, these warnings are unspecific and, in Cartagena's case, grossly unfair. OK, the town has its share of armed guards and police on the streets, but it also feels totally safe. There is a tremendous amount of good in Cartagena. I had the real sense that Colombians I met wanted to change their country's image in the eyes of the world, and that they recognised this will only occur if real changes happen. The festival was part of this, as Vikram Seth pointed out: "It's actually going to make a difference in Colombia. It's a vote of confidence by outsiders." Tourism can play its part in a similar way, especially if directed at the many small, sustainable projects that are currently flourishing in the country.

One such project is Cultura del Mar, a group of four friends who have established an eco-tourism venture in the Islas del Rosario off the coast of Cartagena. The accommodation on their own "island" on Isla Grande (confusingly, patches of land here are called islands despite not being surrounded by water) is simple, a large open-walled thatched hut. The view of the sunrise over the Caribbean is spectacular; the food, wood-fire-grilled freshly caught fish, gorgeous and plentiful; the alarm clock, a pre-pubescent falsetto cockerel, funny, charming and then annoying.

What Cultura del Mar really excels at, though, is transforming snorkelling from the typical casual gazing experience into a stunning, full-blown marine biology expedition. "The butterfly fish", our guide Daniel tells us as we surface above a reef, "is monogamous. When its partner dies, it dies too, from instinct. Just like humans."

I've done a fair bit of diving but that night Daniel takes us on a truly magical night snorkel through the mangroves of the Enchanted Lagoon. Armed with torches and with a soundtrack of Champeta music from a nearby party, we slip into the still, black water to glide alongside the bizarrely alien world of the mangroves. The bulbous, oyster- and coral-clad roots play host to breeding fish and therefore attract larger predators too. It's eerie, strange and brilliant. Daniel catches a puffer fish which blows up in alarm before fluttering off like a tennis ball crossed with a helicopter. The climax comes, however, when we turn our torches off and find ourselves swimming in a constellation of phosphorescent plankton. A clap of the hands creates a galaxy. We dive down and watch our fluid bodies light up in the dark, seemingly composed entirely of brief, living sparks. When we surface we're laughing like children.

As we did when we swam in the crater of a mud volcano 50km north-east of Cartagena. Legend has it that the Volcán de Lodo El Totumo once spouted fire, but a local priest, seeing the 15m mound as the work of the devil, sprinkled it with holy water and turned the fire to mud. I for one am very grateful that he did. Mud-dipping, it turns out, is not only a great laugh, it also leaves you feeling wonderful. Having climbed the rickety wooden stairs, three men guided us into the thick, deep mud. Sinking isn't an option though, even if you try. Which we didn't, preferring to lie back and let ourselves be massaged and floated about the crater like slow, gloopy bumper cars. Fully muddied, women from local families then led us down to the shore of the nearby lagoon where we were thoroughly washed in a way I haven't been since I was three years old. If you fancy staying on for food and drink, there are a few shack restaurants, and you can even take some of the therapeutic mud home with you in a range of recycled plastic bottles.

Back in town the festival of the Virgin of the Candelaria is being celebrated with a Cabalgata, 2,000 ranch horses ridden through the streets of the city. It's an impressive sight, the wagons and carts of beauty queens and kids followed by hundreds of high-stepping, arched-neck Paso Finos, ridden by particularly stylish male and female rancheros. I've never seen someone handle a highly strung horse in high-heels before.

Despite the warnings, then, official and unofficial, Cartagena proved to be one of the most fascinating and engaging travel experiences I've ever had. Both the city and the surrounding area provide more than enough distraction for a longer visit than our nine days. The food is world-class, the people (enhanced and unenhanced) gorgeous and fun, and the town's sense of life and love infectious. We might not have been kidnapped, but a part of us has definitely been held hostage by this complex and beautiful city. I hope, come next year's Hay Festival, we'll be back to haggle over the ransom price.

· Owen Sheers' latest collection of poetry is Skirrid Hill (Seren, £7.99).

Way to go

Getting there

Air Madrid flies Madrid-Cartagena from €490. EasyJet (easyjet.com) flies to Madrid from the UK from £40.98 rtn inc tax.

Where to stay

Hotel Charleston Santa Teresa (hoteles-charleston.com) has doubles from £130 and Hotel Santa Clara (hotelsantaclara.com) from £220, both set within renovated convent. At the other end of the scale is the very good budget Casa Viena, doubles from £6. Cultura del Mar (culturadelmar.com).

Further information

Next year's Hay Festival in Cartagena (hayfestival.com/cartagena) will run from January 25-28.