Angkor Wat, near Siem Reap in northern Cambodia, is the largest religious monument ever built. King Suryavarman II raised it in the early 12th century as the state temple of the Khmer Empire, which then ruled much of Southeast Asia, and dedicated it to the Hindu god Vishnu. Before that century was out, worship here had shifted to Buddhism, and monks have kept the place in use ever since. If you come before sunrise, as most do, you watch the sky behind the towers turn orange and five silhouettes reappear upside down in the reflecting pool. The towers are carved as closed lotus buds, the tallest 65 metres up. To reach them you first walk the causeway across the moat. The whole temple is a model of the universe, so that five-kilometre ring of water stands for the ocean around Mount Meru, the mountain where the gods live. Inside, an 800-metre band of carving wraps the lower gallery at eye level: battle scenes and stories from Hindu myth.
That is still one building, and Angkor is a city of hundreds, spread for kilometres through the forest. You cross between them by bicycle or tuk-tuk, a new tower rising every few kilometres. Two of them draw everyone. Bayon is covered in some two hundred huge carved faces that turn to meet you whichever way you look. Ta Prohm was left to the forest: silk-cotton roots have prised its walls apart and run over the stone until tree and temple are one. Restoration never stops, so you may find a cracked causeway fenced off when you arrive. To Cambodians Angkor Wat is not a historic building but the nation itself. Since 1863 every Cambodian flag but one has carried it: through French rule, the Khmer Republic and the Khmer Rouge, each new regime redrew the flag and kept the temple.