There are two ways up. The old way is a stone stairway of 809 steps, flanked most of the way by statues, so that the climb becomes a slow procession past figures who seem to have been waiting for you. The new way is a paved road, 2.5 kilometres of it, which spares the knees and costs you the ritual. At the summit stands a white-walled temple ringed by shrines and images, and beside it a multi-storied Chinese pagoda whose figurines are made of porcelain - a detail that catches visitors off guard, since porcelain belongs to interiors and this one sits open to the monsoon. Newer wats in the prasat style are still going up near the hilltop, their surfaces crowded with carved nagas and dragons. Next door, Phnom Srah Kmao holds a brick temple and a cave full of bats. The site is in Ko Koh village, in Ko Koh commune, in Santuk District, and it has been accumulating this way for a very long time.

The Cambodian Royal Chronicles put the hill's founding story at around 1496, when King Thommo Reachea I arranged for Buddha relics to be moved out of Preah Thong's stupa at Angkor and rehoused in the village of Khvav Brah Dhatu, near Phnom Santuk. Relics travel for reasons - political, defensive, devotional - and the chronicles do not say which one moved this king. The effect, whatever the motive, was to give a modest rise of rock in Kampong Thom a genuine claim on the sacred geography of the Khmer world.

Later custodians left their names on the place. Preah Thamma Vipassana Kong served as chief monk here in the late 18th century. Older than any monk is Neak ta Kraham Ka, the protector spirit of the mountain, remembered as a man who lived in the feudal era under the ruler Decho Borara'a Thipadei Meas. A neak ta is a local guardian spirit, one of the older powers that Cambodian Buddhism absorbed rather than displaced, and their shrines sit comfortably beside the Buddhas. Seven monks perform the worship on the hill today.

In the autumn of 1971 this hill was a military objective. Operation Chenla II ran from 20 August to 3 December, the Khmer Republic's attempt to reopen Route 6 and push the People's Army of Vietnam back out of the country's centre. The advance began well. Government forces retook Barai on 26 August and Kompong Thmar on 1 September, and the 5th Brigade Group fought its way to Phnom Santuk and took the hill. On 25 October the phase was declared complete. Phnom Santuk was the furthest north-east the American-backed army ever reached.

It held for a day. On 26 October the PAVN 9th Division, reinforced by the 205th and 207th Regional Regiments, came down on the positions strung along Route 6. Battalions were surrounded, isolated, cut off from relief, and by early December ten of the Khmer Republic's infantry battalions had been destroyed or dispersed, with equipment losses equivalent to ten more. Casualty estimates vary wildly and none of them are comfortable: North Vietnamese accounts claimed 4,500 enemy dead, while the official PAVN history put the figure at 10,000 killed or scattered. Whichever is nearer the truth, the defeat left the Republic's army weakened both militarily and psychologically, and it never regained the initiative.

Nothing about the mountain is finished. Construction continues near the hilltop. Concrete Buddhas join stone ones under the trees, the newer ones easy to tell apart and nobody much troubled by the difference. The Phnom Santuk Resort offers nature-based tourism at the foot of a pilgrimage site, an arrangement that would look incongruous in a museum and looks entirely normal here.

Pilgrims come to make merit. Visitors come for the climb and for the view out over the Kampong Thom plain, which from 207 metres runs flat and green to every horizon. Both are accommodated without much fuss. That is the durable thing about Phnom Santuk: kings moved relics to it, a civil war swept across it, construction crews are still at work on it, and the stairway is still climbed one step at a time. The carved sleepers under the southern peak have outlasted every army that ever came for the hill.