Visitors to the ruins of Urasoe castle get no clues, as they stroll through lush tropical vegetation, gaze at a sparkling ocean a few hundred yards away, or stop to take in the tranquillity of a Buddhist shrine, that 75 years ago hell was unfolding where they stood.

The ridge, now remade as a city park that forms a small oasis of green in a sprawl of encroaching suburbs, was at the heart of the brutal battle for Okinawa. It was also the site of one of the most extraordinary acts of heroism ever recorded by the US military.

In May 1945, combat medic Desmond Doss spent hours at the top of the ridge, unarmed and in the line of direct fire, after ignoring orders to retreat. Instead, he painstakingly winched 75 injured men down a steep cliff to safety, only following himself when all the others had been evacuated.

That astonishing bravery is celebrated in the new Mel Gibson film Hacksaw Ridge. The title comes from the nickname US soldiers gave the scrubby hill that came to define their war on Okinawa, at least in the popular imagination back home in America.

As fighting surged back and forth over the ridge for several days, about 2,500 Americans lost their lives and nearly twice as many Japanese troops, said Chris Majewski, a former marine who, as director of the island’s battle of Okinawa museum, leads regular battlefield tours of the site.

Up close, it seems almost unimaginable that so many lost their lives battling for control of this small outcrop. Gibson apparently felt the same, because the Hacksaw Ridge he re-created in Australia is a far larger, more dramatic peak than the Okinawan hill.

Desmond Doss on Okinawa in 1945. Photograph: Courtesy Desmond Doss Council

He notes with slight frustration that there is no record that Gibson ever visited the site for research purposes, and the film set certainly appears to owe a large debt to someone’s imagination. An uneven face of limestone coral, approximately 30ft high, has been turned on screen into a towering cliff which is three times as tall. The hilltop itself, a scrap of land barely 150ft long, has also been expanded to more than double its actual area.

“The cliff in the film looks like the wall from the Game of Thrones set,” said Majewski, adding that visitors who have seen the film are set for initial disappointment when they arrive at the actual Hacksaw Ridge. “I always get that [response]: ‘It’s so small!’”

Still, being at the site where Doss rescued so many lives is usually profoundly moving for visitors. Majewski can point to the spot where the medic winched down the wounded, marked by a lump of coral rock, because Doss himself identified it on a return to Okinawa decades later.

President Truman awards Desmond Doss the Congressional Medal of Honour in October 1945. Photograph: Bettmann/Bettmann Archive

Majewski, who met the medal of honour winner when he returned, is particularly happy his story is being shared with a new generation. “[The film] puts into the spotlight the story of a man who would never seek the attention for himself,” he said.

Okinawan authorities have not promoted the site, limiting restoration and signposts to prewar heritage, perhaps because the island is still unpicking the bloody legacy of a conflict that many felt was imposed by rulers 930 miles away in Tokyo. Up to a quarter of its population died in a three-month battle – known as the “rain of steel” for the intensity of the fighting – 90% of the buildings were razed and fields were left laced with mines, corpses and spent ammunition.

Disputes over how aspects of the conflict should be described in school textbooks rage on today, particularly the mass suicide of civilians under duress from the military, and there is resentment among some on the island that it still hosts a chain of American bases 70 years on.

Majewski addresses the wider context of the conflict in his tours and hopes to bring greater understanding of the island and its people. “There is a different appreciation for the island and the people who live here,” he said. The tours also serve as a warning to younger people, brought up on violent video games like Call of Duty, against being seduced by the Hollywood vision of war.

“When you see the movie, then see the actual site or vice versa, it makes you wonder what else they took creative licence with, what else did they exaggerate?” he said. “So maybe it makes people pick up a book and learn the true story and helps keep the real history alive.”