The bridge runs about 87 metres long and roughly 14 metres wide, carried on twenty-one narrow arches. Except they are not arches in the sense a Roman engineer would recognise. There is no keystone and no wedge-shaped voussoir. Each opening is a corbel arch: courses of stone stacked so that every layer projects a little further inward than the one below it, until the two sides meet at a point. It is a technique that works entirely by dead weight and friction, which means it needs enormous mass above it to stay put — and mass is exactly what Spean Praptos has. The blocks are laterite, the rusty, pitted ironstone that hardens in air after being cut soft from the ground. It is ugly at arm's length and close to indestructible in bulk.

Attribution goes to Jayavarman VII, who ruled from 1181 to 1218 and whose building programme was among the most ambitious of any medieval monarch anywhere. The Bayon and Ta Prohm are his. So are hospitals, rest houses and a network of causeways radiating from Angkor across the empire — infrastructure as an instrument of rule. Spean Praptos is one of the few Khmer bridges from that era still standing, but it was never a solitary marvel. Spean Thma survives on the former course of the Siem Reap River between Angkor Thom and the East Baray, Spean Memai stands within the Angkor complex itself, and the same corbelled pattern turns up at points across the old empire. What is unusual about the Kampong Kdei bridge is its length, which is why it has so often been described as the longest corbelled stone-arch bridge in the world.

A bridge in the Khmer world was not merely a way across water. Along each side of the deck runs a balustrade carved as a naga — the multi-headed serpent of Khmer cosmology, its body forming the rail and its hoods rearing up at the ends. At the centre of each balustrade there would originally have been a seated Buddha, and guardian figures marked the boundaries at either end. The naga is a water spirit, and putting one along both edges of a river crossing was not decoration so much as insurance. Some of the sandstone has been lost, some replaced, but enough remains that the intent is unmistakable. Villagers still leave offerings here, and people come to pay respects to ancestors on a structure their families have crossed for generations.

The bridge absorbed a major restoration campaign between 1964 and 1966 under French archaeological supervision. But the road it carried became National Road 6, the main artery between Siem Reap and Phnom Penh, and the vehicles using it grew steadily heavier than anything its builders had imagined. By 2005 the Ministry of Public Works and Transport, with the approval of the APSARA Authority, had built a short bypass and a new parallel bridge alongside the old one, closing Spean Praptos to motor traffic for good. Highway traffic now goes around. Spean Praptos itself carries pedestrians, motorbikes and light local movement — still in use, still part of the town, but no longer asked to hold up a freight economy.

Most of what Jayavarman VII built has been reclaimed by forest, cut apart by looters, or propped up by conservators. This bridge simply kept working. Part of that is the material: laterite weathers to a crust that resists erosion rather than accelerating it. Part of it is the geometry, since a corbel arch under compression only gets more stable as load presses down. But the larger part is that people never stopped needing to get across the river at this exact spot, and a thing in continuous use tends to get repaired. Eight centuries of oxcarts, bicycles, ox-drawn timber, army convoys and motorbikes have polished the laterite where feet and wheels pass. The bypass was not a retirement. It was maintenance.