"It's going to mean some sacrifices," Burnham said. "People aren't going to be able to do some of the things, in an unregulated way, that they've been permitted to do in the past."
Numerous armies have barged through the city-state founded 1,200 years ago. Its temples were abandoned to the jungle during almost half that period. Angkor Wat suffered its worst damage when Khmer Rouge fighters looted it in the late 1970s as they were committing mass murder in the name of an agrarian revolution.
Foreign donors and governments, led by the U.S., France and Japan, have spent as much as $50 million over the last 15 years to repair the scars of time and abuse. But the work is far from finished, and new threats are building.
Sokimex Group, which has used its connections with Cambodian Prime Minister Hun Sen to become the country's biggest company, plans to build a 900-room hotel and spa, with shopping mall, water park, slot machines and conference center, on a 56-acre site in Siem Reap.
Sokimex also controls the ticket concession to Angkor. Passes cost $20 a day, $40 for three days and $60 a week. It's small change for a company that deals in oil, gas stations, pharmaceutical products, garment making, property development and luxury hotels and resorts, in addition to running an airline.
Sokimex's share of the admission take is set by a contract with the government, and Burnham said it leaves most of the profit in the company's hands. One-third of the revenue is supposed to go to Apsara, a Cambodian agency set up by royal decree to preserve the Angkor sites and manage development.
But some people dispute the ticket sales figures, saying Apsara -- which takes its name from the heavenly nymphs of Hindu and Buddhist mythology whose bare-breasted figures adorn the Angkor temple walls -- gets enough only to cover basic expenses.
"Apsara has virtually no money for conservation," Burnham said. "All of the conservation at Angkor is being done through international assistance."
The effect of millions of feet pounding on Angkor Wat's steps and floors already has led officials to close some areas. The towers, the tallest of which rises 213 feet, are off limits because the constant wear and tear made the structures unsafe.
A first step toward reducing congestion could be as simple as insisting that visitors walk through Angkor Wat in the same direction, from beginning to end, Burnham said.
She also wants to see Cambodian officials set time constraints on tickets for the busiest of Angkor's temples, to limit pressure during peak hours.
The day may come when a strict quota is placed on the number of visitors allowed at certain monuments, Burnham said. But O'Reilly hopes to avoid that by persuading tourists and their guides to make better choices.
O'Reilly is deputy director of the Greater Angkor Project, a team of researchers at Australia's University of Sydney who in recent years have discovered how vast ancient Angkor was by studying images taken by NASA satellites and an ultralight plane.
Their theory is that the city's 15th century collapse occurred largely because people neglected their environment, cutting down too many trees to expand rice paddies, causing waterways to fill with silt.
If they're right, it's a cautionary tale for the 21st century, as overdevelopment threatens the magnificent buildings and art that ancient Angkor left behind.