“The jewel in the crown” is how Sue Benson describes Greenwich Park, with its commanding views across the River Thames to the new high-rises at Canary Wharf, to St. Paul’s Cathedral and the Millennium Dome. The park is home to the Royal Observatory, where international delegates agreed in 1884 to establish zero longitude — and thus the place where time begins.

All this spectacle and symbolism will be the backdrop in two weeks for Olympic equestrian events, and Benson, 60, has been charged with designing a 3.5-mile obstacle course here for what she says is “the most dangerous sport in the Olympics”: Three-day eventing. Doing so in a 183-acre royal park that is also a World Heritage Site has involved finding ways to protect rare acid grasslands and the root spans of centuries-old trees from hundreds of hooves. It’s meant respecting the rights of dog walkers and cyclists, as well as working near Roman remains where Benson plans to build a particularly testing combination of jumps commonly called a “coffin.”

A big challenge, she said, is to create a course on the park’s precipitous slopes that will test the mettle of strong teams from countries such as the United States, Britain, Germany and New Zealand, without endangering horses and riders considered long shots .

It’s been a process of compromise and negotiation that not only exemplifies the difficulties of staging the Games in the heart of a vibrant capital city, but also reveals the complications of a sport in which the Olympic standard is not simply to be “faster, higher, stronger.”

In fact, Benson said last week, because of the hilly terrain here the Olympic fences won’t be as numerous, and some won’t be as big, as those top event horses face elsewhere.

“We’re way under the maximum,” the former international event rider said. Her course will be designed to find a true champion, but in comparison with the most demanding courses in the world — the Rolex Kentucky or Britain’s Badminton — she said, “We’ve had to back off a bit.”

Building a safer course

Three-day eventing dates from the days of the cavalry, when chargers were expected to perform on the parade ground as well as the battlefield. This exacting test of horse and horsemanship became an Olympic sport a century ago, and today is one of the rare contests in which women and men compete against one another.

During the first of the event’s three phases — dressage — judges score horses on their obedience and finesse in executing of a series of gymnastic movements in an arena.

The cross-country phase follows — a test of speed, endurance, courage and jumping ability over the kinds of fixed, solid fences, banks and ditches riders might encounter in the countryside. Horses earn penalty points for refusing to take a jump, for example, or dodging an obstacle, as well as for failing to finish the course within the allotted time.

A day after the grueling cross-country, show jumping proves the horse’s fitness and accuracy, but it isn’t intended to be the defining factor.

Make the cross-country course too simple, then, and the Olympic medals will be awarded based essentially on dressage scores. Make it too difficult, and you risk discouraging or endangering horses and riders.

Benson wants to see as many horses as possible complete her course successfully, but given the hills, she said, “I don’t think many horses will get around within the time.” She said she can’t recall an Olympic course that has “such demanding terrain and restricted space.”

She is building several obstacles with alternative routes — one direct path over challenging jumps, and a second, longer route over simpler fences. Riders who are not in a medal position can choose the slower, safer option, even if that means incurring time penalties. “But will they make that choice?” Benson asked. “That’s what I can’t control.”

A series of rider deaths — many in the United States and Britain and often caused by a horse somersaulting over a fence and crushing its rider — led the FEI (Federation Equestre Internationale), which governs international equestrian sports, to start monitoring the sport, including falls. Based on data from 2004 onward, the FEI has created a risk management plan aimed at increasing safety by such measures as preventing reckless riding and improving fence construction.

The goal, said Catrin Norinder, the FEI’s director of eventing and Olympic sports, “is to reduce horse falls by 10 percent by the end of 2013.”

Some of Benson’s jumps will be supported by pins designed to release under the force of a horse that is beginning to somersault in order prevent it pitching further forward.

“Safety is as high as it can be,” Benson said.

She walked through a former boating lake, pointing out the giant platforms from which horses will jump into and out of the water.

“To win gold,” Benson reflected, eyeing the intimidatingly high and narrow exit on the direct route, “they’re going to have to do this.”

Respecting the park

The details of the individual jumps are secret, under construction at five or six workshops around the country, but Benson explained she has tried to use them to “tell the story of England.” The boating lake will be transformed to look like an industrial canal. In another area she’s creating a fanciful river scene, enhanced by hundreds of wildflowers she’s been cherishing at home. Think “The Wind in the Willows.”

The path that the horses will take has been obvious for months, though — a swath of brilliant green, about 10 yards wide, that crosses the prime meridian three times on its route around the park. On one of those crossings, Benson plans a jump composed of 22 clocks, each set to the time in the capital city of one of the 22 competing nations.

Keeping that grass in tip-top condition was a constant challenge until the park was closed to the public a week ago. Dogs would “come along and dig it up,” Benson said. Although the most vocal opposition to using the park for the Olympics has quieted, some local residents, including Ellie Ceri, a professional dog walker, said she and other people who bike and run there are feeling “a bit jaded” by the disruptions and now the closure.

Even fertilizing and reseeding have been controversial, because the track runs through some patches of acid grassland, a heath-like habitat, rare in London, that supports insect life, spiders and wildflowers. Liz Coyle, chairman of Friends of Greenwich Park, said her volunteer group’s role has been to “keep a jolly close eye on what’s going on,” and she’s satisfied that no permanent damage will be done.

Spiking the ground in order to aerate the course raised a flurry of worries about disturbing archaeological artifacts in a park that boasts Anglo-Saxon tumuli and a Roman temple — a prospect Coyle dismisses as “extremely slim,” adding that an archaeologist is on site, and that a survey of one area close to Roman remains produced nothing of value, just candy wrappers.

In the famous flower garden, horticulturalists agreed to move some beds to allow horses to gallop past 50 hanging baskets and over Benson’s huge, colorful jumps. She will prune some of the lower tree branches so they won’t clip riders’ helmets. But she won’t cut any trees down.

It would take an event of even greater national importance than hosting the Olympics to get the go-ahead to lop off whole limbs — acts of war, not Games. During World War II, antiaircraft guns were stationed nearby to shoot down German planes flying up the Thames to the capital on bombing raids, according to Steve Hunnisett, who leads guided walks describing London’s wartime history. Some trees were cut back to widen the field of fire. Seventy years on, the story of that extraordinary time is told in the trees’ misshapen canopies.

But after 75 event horses and tens of thousands of people leave here, Benson and her crew have promised, she said, “to return the park in the condition we got it in.”