It was only on my third or fourth day at the temples of Angkor Wat that I started to appreciate how they are more than just a tourist attraction - they are at the heart of Cambodian national pride and identity. The level of technological skill and organisational ability required to construct the city of Angkor is evidence of the level of advancement of the ancient Khmner civilisation - according to contemporary accounts (from around the 13th century), Angkor was one of the most magnificent cities in the world. Today, walking around the ruins, it becomes easy to understand why the modern Cambodians draw a sense of prestige from the fact that they are the descendents of a great people. And what also starts to become clear is the degree of dislocation and devastation that must have been felt when the Khmer Rouge came to power and declared "Year Zero": history was dead, everything that went before was to be destroyed. The past was forbidden.

The Choung Ek Killing Fields, a few kilometres outside Phnom Penh, is only one of the hundreds of mass grave sites that were found all over the country after the defeat of the Khmer Rouge regime. The remains of approximately 9000 people were found there. At the centre of the site today is a memorial - designed as a modern version of a traditional Buddhist stupa - where the skulls from some of the victims are interred in a glass case. A tourist guide pointed out how the method of killing could be determined from the form of damage to the skull: a long slit indicated an axe blow; a small hole, a bamboo spear; a large portion of the skull missing, a shattering blow from a blunt instrument. Most of the graves at Choueng Ek have now been excavated, and all that remains are shallow depressions in the ground. But pieces of bone, teeth, and clothing fragments are still scattered about the site, and next to many of the graves a sign nailed to a tree gives more information: some of the graves contained only headless corpses; some just those of women; others, babies whose heads had been smashed against a nearby tree. I watched as a couple of Japanese tourists stood under one of these signs and got their friend to take a photo of them grinning and waving. Maybe it's a cultural difference, or maybe some people just don't get it.

Back in Phnom Penh, the Khmer Rouge story continued, at the Tuol Sleng prison museum. Until the fall of Phnom Penh this was a school, but with the Khmer Rouge takeover the buildings were converted to prison cells and interrogation rooms. Prisoners were tortured so that they would implicate their friends and relatives as accomplices in their alleged crimes - in this way, the Khmer Rouge commanders were able to wipe out entire extended families. When the prisoners could give no more names, they were taken to the killing fields at Choueng Ek. Every prisoner was photographed when they entered Tuol Sleng, and today the rows of faces stare back at visitors to the museum. A few of them show expressions of fear, but more often there is simply a sadness, an understanding that something terrible is happening to their country. The age ranges of the prisoners vary from the elderly to children aged about 10 or 12. The Khmer Rouge liked to recruit children, since they were easily indoctrinated with the revolutionary ideology. Periodically, there would be purges of entire Khmer Rouge divisions that were seen as a threat to the leadership of Pol Pot, and the young soldiers would end up in Tuol Sleng.

The photographs provided a human face to the suffering, and made the Tuol Sleng museum a much more disturbing experience than the killing fields. In another room of the museum was an exhibition of more recent photographs, of survivors of Tuol Sleng - both prisoners and guards - who had rebuilt their lives after the fall of the Khmer Rouge. Next to each picture was a short quote from the person featured; the prisoners looked for explanations of what had happened, while the guards talked about getting over the past, carefully avoiding any mention of their degree of personal involvement. A short film about the prison featured an interview with a former guard, who admitted killing prisoners with an iron bar, and throwing their bodies into a pit. It did not seem difficult for him to talk about the events, and he even smiled as he recollected how "someone told me to do it, and I did it." Maybe the smile was a defence mechanism, maybe not. Tuol Sleng reminds us that, just like in Nazi Germany, the mass killings were not the work of a few derranged individuals, but of many ordinary people - some of whom survived and went back to living ordinary lives.

Of course, Phnom Penh has much more than the memorials to the Khmer Rouge years. The esplanade along the river is beautiful in the evenings, and the Royal Palace and National Museum exhibit the gems of traditional Khmer artwork. But images of Tuol Sleng and Choueng Ek were still in my mind as I boarded the fast boat up the Tonle Sap river for Siem Reap, the modern town closest to the ancient "lost city" of Angkor. A couple of hours into the journey, the river widens into the Tonle Sap lake, which was at the heart of the Ankorian kingdom. The flooding of the lake irrigated the rice fields, and there were abundant fish stocks to provide a source of protein. The resulting food surplus enabled the Angkorian empire to support the thousands of specialised workers - stone-workers, architects - necessary for the construction of the great city.

After arriving at Siem Reap in the afternoon, I followed the advice of Lee, my motorbike driver, and let him drive me to the base of the Phnom Bakheng hill, to watch the sunset. To get to the hill we drove past the entrance to Angkor Wat, and I had my first sight across the moat to the famous towers, which are featured in the centre of the Cambodian national flag. Somehow it was difficult to appreciate that I was actually here and seeing these things - it felt like a film was being projected at the side of the road, and we were riding past it. The thing that immediately struck me was the sheer scale of the thing, and it seemed to take forever to ride from one side to the other. But we reached the base of the hill, and I climbed to the top to find it swarming with tour groups, who all know that it's a classic sunset spot. The views over the plains, and through the trees to the towers of Angkor Wat in the distance, were impressive, but it was just too crowded for any real atmosphere, and I left before the sun went down.

Over the next few days I gradually learned how to avoid the tour groups. It's not difficult, since they all follow more or less the same schedules: the classic "circuits" connecting the most popular temples, which were first worked out in French colonial days when visitors did the route on elephant-back. On my first day, however, I decided to follow the crowds and get the main sights out of the way, so that I could return to my favourite spots when the crowds weren't there. I started at the walled city of Angkor Thom, passing through a gate above which enigmatic stone faces stare out in the four cardinal directions. On past the Bayon, the remains of the old Royal Palace, the beautifully preserved stone carvings on the Terraces (where it is thought that military parades were held), then out of the city via the east gate to some smaller temples, including Ta Phrom, which has been left partly unrestored, and still has very photogenic trees growing out of the ruins.

My final stop of the day was Angkor Wat itself - perhaps the high point of Khmer architectural and artistic achievement. The walk across the moat towards the centre of the temple is meant to represent the spiritual journey of the soul through life and death, and the lengths of the walkways connecting different sections of the temple are thought to represent the ratios of different epochs in Hindu cosmology. After crossing the moat, the covered galleries are decorated with bas-reliefs, depicting the Hindhu creation myth, epic battles, the fate of souls in heaven and hell. Further in are amazingly detailed carvings of apsaras, the dancing nymphs who were supposed to have been born out of the churning of the cosmic ocean, and who were obviously a favourite subject of the Khmer stone-workers - there are just so many of them, and they are everywhere.

The climb up narrow and steep steps to the higher levels of the temple supposedly represents the ascent of the soul up mount Meru, the cosmic mountain that is the centre of the universe and the abode of the gods. Appropriately, the holiest sanctuary is at the highest level. Just as the soul's journey is not meant to be easy, the stone steps are dangerously steep; when I reached the top, tourists were forming queues to start the risky descent back down again. When I looked down I could see people streaming out of the temple, across the walkways and out of the main entrance, probably on their way to watch the sunset at Phnom Bakheng. Before long, the highest level of Angkor Wat became almost deserted, and the atmosphere felt calm and slightly mysterious as the red glow of the sun illuminated sections of the stone corridors. It had been a long day, but exhilarating. The only way to really describe the experience is to say that all of the travellers tales, all of the hype about Angkor is true. It really is as amazing as everyone says.

Over the next few days I explored some of the more outlying temples, and returned to others at different times of the day, both to avoid the tour groups and to take photos under different lighting conditions. And every time I returned to a temple there seemed to be more to see, different aspects to be discovered. At first, it is the sheer scale and grandeur of the larger temples that is impressive, but as you look more closely it is the smaller scale details that really capture the imagination. The amount of stone carving is overwhelming - every inch of exposed stone seems to be carved: the walls, ceilings, pillars, sometimes even the floors. The carvings might be elaborate depictions of Hindu gods, or hundreds of dancing apsaras, or sometimes just geometrical patterns to fill up the space. Everything is carved - the ancient Khmers were not just great artists, but they also knew how to crank it out!

By the end of my stay at Angkor, I wasn't so much templed-out as apsara'd-out; when I closed my eyes I could still see the outline of an apsara, as if it was burned onto my retina. Instead of returning straight to Phnom Penh, I decided to take a detour via Battambang, since I had a vague plan about taking the train from there back to the capital, along the only section of railway still operating in Cambodia. (Eventually I wimped out of doing the train ride, after hearing too many reports of nightmare 20-hour journeys and bits of the train falling off en route.) The boat ride west to Battambang was advertised as taking 5 hours, but ended up taking more than 8. The water level of the river was low, so about half way into the journey we had to transfer from one big boat to two smaller ones, which were capable of navigating around the exposed rocks and sandbars. Battambang turned out to be just far enough off the main Phnom Penh-to-Siem Reap tourist trail to have a more authentic Cambodian feel, but still with enough visitors to justify a riverside bar that served chip butties.

Close to Battambang, there is not a killing field, but a killing cave. Khmer Rouge guards would make prisoners kneel, with their hands tied behind their backs, at the edge of an opening in the top of the cave, smash their skulls with a heavy iron bar or rifle butt, and push them in. In 1979, soldiers of the Vietnamese-backed army who defeated the Khmer Rouge found the bones of hundreds of victims piled up in the cave. I visited the cave with Chhay, my motorbike driver and guide for the day, who was 8 years old when Pol Pot came to power. Chhay said that he was half Chinese, and had only survived those years because he could speak Khmer without a noticeable accent. He said that he had fled from Cambodia to work in Thailand, before returning a few years ago. He did not volunteer any information about what had happened to his family, and I didn't ask him.

The cave was formerly home to a colony of swiftlets, but they were hunted and their nests taken to make soup. They have left their mark in the form of hundreds of holes - nesting sites - in the soft earth on one side of the cave. As my eyes were adjusting to the gloomy half-light, the holes formed themselves in my vision into grotesque shapes, skull fragments, eye sockets and mouths twisted and distorted. I walked further inside, to a shrine in memory of the victims, next to which was a wire-mesh casket containing the skulls of some of them. Their relatives come to pray at the shrine, and leave offerings of rice or incense outside the casket. Sometimes, if their loved one had liked to smoke, they push a lit cigarette through the holes in the wire-mesh, into the mouth of one of the skulls. Each person who visits the cave ensures, in their own way, that the dead are not forgotten.