Until then it had been the perfect night in the Caribbean. I was sitting on the white powder sand of Colombia's Tayrona National Park. Waves crashed unseen nearby and the moonlight sparkled in broken patterns on the water. My senses must have been dulled by one too many Mohitos earlier - because I was unaware of four large figures approaching across the dark sand. They surrounded me before I knew it. Gut reaction and a blind surge of adrenaline made me instantly spring to my feet - ready to flee or defend myself. A micro-second later, my brain processed that these heavily-muscled men were wearing uniforms; two of them military and two of them police. My body flushed with momentary relief that it wasn't thieves or bandits come to kidnap me. Then they began shining flashlights in my face and barking questions in gruff Spanish.

When one of the military types roughly snatched my backpack and started rifling through it, I understood what they were looking for. Cocaine. I should have realised it sooner. This far Northeast tip of Colombia is a major Cocaine-smuggling region, where small vessels arrive in the dead of night to be loaded with their illicit cargo bound for the United States. Employing my basic Spanish skills I repeated "No drogas! No drogas para mi" (No drugs! No drugs for me) several times, which was roundly ignored until they'd finished searching my bag. Eventually they were satisfied that I had none of Colombia's most famous export on my person. Then, without so much as a "Buenas noches," they ordered me off the beach and dissolved into the darkness whence they'd come.

The next morning, the unpleasantness of the previous evening's drug squad encounter was fading fast. Any lingering fear was being driven from my mind by the sheer beauty of my tropical surroundings. Every one of us has a fantasy of a Caribbean beach paradise imprinted our minds. This mythical Caribbean vision has been nurtured by novelists and Hollywood film directors and grown plump and magical in our imaginations. It's the ultimate prototype of perfect crescents of sand overhung by palm trees. It is a deep-seated fantasy of impossibly clear turquoise waters lapping the shores of a jungle filled with the chattering of monkeys and exotic birds. Sadly, many cynics believe that such a place doesn't exist any more. Well, I can confirm that it does exist. It is Colombia's Tayrona National Park.

Tropical wildlife fills the steamy jungles fringing these postcard perfect shores. I saw thumb-sized Hummingbirds buzzing between flowers extracting pollen at the edge of reed-fringed lagoons. Upon entering Coconut palm groves dozens of royal blue coconut crabs retreated from my feet while wielding their pincers threateningly. Meanwhile, high above in the canopy, white-faced Spider Monkeys leapt playfully between branches or hung by their tails watching me far below with curiosity. Each idyllic bay concluded in another headland of spectacular weather-beaten boulders. Clambering over these vehicle-sized, smooth granite megaliths, I would often disturb rainbow-hued lizards sunning themselves on high vantage points overlooking yet another perfect crescent bay.

Although the wildlife inhabiting Tayrona National Park is so dense I was nearly tripping over the creatures, I encountered few humans to ruin the serenity. On occasion, I walked for an hour or more without seeing another soul. Its status as a national park means Tayrona is protected from commercial development and therefore it's beauty remains unmolested. There are only three 'Fondas' (basic restaurants) spread out over a few kilometres, where cheerful locals sell cold drinks and simple meals that usually consist of the ubiquitous combination of rice beans and chicken. If you wish to stay the night you'll have to rough it because there are no rooms for rent. Your only options are to pitch a tent or hire one of the hammocks strung in long rows under palm-roofed shelters.

250km west along the sparkling Caribbean coast lies the colonial Spanish city of Cartagena. It's hot here. Not your average hot, either. Cartagena is the hottest place I've ever been. At 9am on a July morning it was already pushing 37 degrees celcius, with almost 100% humidity – and I was soaked in sweat. Panting pitifully, I followed the locals' example and criss-crossed the rough cobbled streets to walk in the shade of tallest buildings. These ornately terraced and brightly painted Spanish colonial structures are impeccably preserved and the main reason tourists visit this historic seaside city. But in my hot-and-bothered state I was having great difficulty appreciating Cartagena's world renowned architecture. In fact, it only meant one thing to me. Shelter from the sun. So, I ignorantly scurried along narrow cobblestone streets, rushed beneath 16th century stone archways and wooden balustraded balconies, barely registering the stunningly preserved history all around. I was seeking the bland modern design of one particular building: a supermarket – and the air-conditioning I knew it offered. Sightseeing would have to wait until it was cooler.

Cartagena was designed not only to provide its citizens protection from the brutal Caribbean sun, but also the ruthless Caribbean pirates. The entire Old City is encircled by 11km of massive stone walls that are four metres thick in places. Defensive guard towers, canons and sentry boxes stud the rim of this wall – particularly on the exposed northern seaward side.

Founded in 1533 by Spanish commander Pedro de Heredia, Cartagena quickly grew to become a major trading port for the Spanish empire. From these shores the gold and silver wealth of Colombia and Peru was spirited away aboard galleons bound for Europe. Cartagena's fast rising wealth and fame soon made it a juicy target for French and British pirates and privateers, who took turns ravaging, looting and setting fire to the city. After a particularly brutal attack by legendary British pirate Sir Francis "The Dragon" Drake in 1586, in which he destroyed a quarter of the city and then held it to ransom, the Spanish crown began pouring millions into the defence of their prized port. After more than 200 years of engineering work costing billions of modern day dollars, Cartagena was considered impregnable. Today, all of the fortifications remain perfectly intact.

Great stone fortresses outside the Old City's walls guard the rear and side approaches from would-be pillagers sailing up the lagoons that encircle the island-city. The most important of these is Castillo de San Felipe de Barajas. Although at a distance it appears to be a simple truncated pyramid, this imposing stone structure is a miracle of defensive design. Drawbridges stand strong, canons bristle from every vantage point and the bowels of the pyramid are honeycombed with a labyrinth of tunnels. Thankfully the whole fortress is open to visitors to explore. At sunset you can peer out narrow slits in the highest guard towers (designed for firing an arrow, while remaining protected) that overlook the walled city and imagine life here 400 years ago. This isn't much of a mental leap because Cartagena's architecture has remained virtually unchanged over the centuries. It's easy to understand why such an immaculately preserved colonial gem was protected as a UNESCO World Heritage Site in 1984.

Most of Castillo de San Felipe's claustrophobia-inducing tunnels are open to visitors, but not many people choose to explore them. And it's not hard to see why. Barely more than five feet high and just wide enough for two people to scrape past each other, these wet passageways angle downwards at a steady 30 degrees. Ignoring my pathological aversion to dark, confined spaces, I descended into the belly of the beast.

If I thought it was hot outside, the stagnant air beneath this fortress was like being in a brick-lined sauna. The air was viscous and hung like cobwebs in dank corners. Stale water dripped from the stonework and I could hear the eerie distorted echoes of other subterranean explorers as I plunged deeper into the earth. Weak light bulbs spaced 30m apart, emitted a pale and flickering yellow glow that failed to inspire any feeling of confidence in my descent. After a few minutes penetrating the furthest corner of this hellish catacomb I began to lose my nerve, feeling the weight of millions of tons of stonework above me. When my path was partially blocked by a pile of fallen masonry I took the hint and retreated towards the light.

The Spanish founders of Cartagena might have been inspired defensive tacticians, but they were lousy city-planners. You see, each street block in the Old City bears a different name. Every time you pass another side road, the name of the street you're following changes. It's maddening. In the space of 400m, Calle don Sancho becomes Calle de la Iglesia, then Calle de los Santos de Piedra, then Calle del Landrinal before finally transforming into Calle san Claver. Trying to find a specific location without a detailed map is an exercise in futility.

One day I was grappling with the insanity of Cartagena's street system while searching for reputably the best seafood restaurant in the Old City. A horse and cart rattled past inches away from me in the narrow alley. Slightly startled, I glanced up as the driver flicked his whip behind the ear of his prettily adorned horse. I momentarily caught the eye of a local man in his 50's. He grinned charmingly and gestured for me to approach. I knew there was no escape. Although my Spanish is passable, he insisted on speaking halting English. “Where are you from?” “Oh Nueva Zelanda! (New Zealand) Fantastic!” “My name is Victor - welcome to my wonderful country of Colombia”. After many pleasantries and lots of toothy smiling and nodding designed to show his sincerity, he got down to business. “You need some Colombian Pesos? - I give you my best rate, my friend.”

Of course, I knew in all likelihood this was a scam designed to part me from my money. Cartagena is notorious for them. These black-market cambios (money changers) offer you an impossible-to-refuse rate for changing American dollars into the local Peso currency, then somehow swindle you out of most of it. The scam usually involves either fake bank notes (which Colombia is awash with) or some deft sleight of hand that leaves you short-changed.

Even forewarned of these tricks, I was entranced by Victor's charming manner and smooth technique. It also helped that he was offering me a rate 25 percent better than the official rate. Half of my brain screamed “Don't be an idiot! This is too good to be true!” while watching the other half of my brain open my mouth and agree to change US$100. Victor set off at a furious pace through a twisting series of labyrinth cobbled alleyways as I struggled to keep up, all the while desperately trying to figure out how to renege on my agreement to buy his Pesos. He stopped suddenly, signalling subtly to another man standing in a nearby stone archway. The stranger quickly walked towards us, stuffed a huge wad of banknotes into Victor's outstretched hand, then continued on his way without breaking his stride or uttering a word. If I needed confirmation on the shady nature of this transaction, I had just received it. With a hasty apology, I got out of there as fast as possible, with Victor calling angrily after me.

That was my lesson on Colombia's modern day pirates. They no longer threaten the walls of Cartagena in their galleons; now they roam the cobbled streets looking for gullible gringos.