The hostel in central Seoul has a lot to recommend it. The rooms are tidy and affordable enough for K-pop fanatics on a budget and families in need of lots of space on vacation. It’s perched at the base of Namsan, the scenic, leafy mountain peak in the heart of town. There’s even a rooftop with panoramic views of the city.

Just don’t try to go to the basement.

Namsan, with its winding trails and springtime cherry blossoms, has long been a top destination for tourists in Seoul. Not long ago, though, “going to Namsan” meant something different, something sinister.

The phrase was typically used during South Korea’s postwar authoritarian years as a euphemism for bringing pro-democracy protesters to the Korean Central Intelligence Agency headquarters and interrogating them. Torture was common, and one of the preferred venues was the basement of the Seoul Municipal Youth Hostel, which once housed the K.C.I.A.

The building now stands as a symbol of the country’s tangled relationship with the past and the present, though its shameful reputation hardly seemed to register with guests who milled about in the lobby on a recent afternoon, requesting bathroom towels from reception and taking cheery group photos.

The pleasant, shaded path toward the hostel is lined with subtle, easy-to-miss nods to Korean history. A small plaque on the ground is engraved with the words “Trail of National Humiliation,” a reference to the nearby location where the Japanese resident-general of Korea lived during Japan’s occupation of the Korean Peninsula, which ended in 1945.

The Sixth Bureau Building, another K.C.I.A. torture site, is also in the vicinity. That building, with its distinctive red exterior, has a mock interrogation bunker that visitors can view as an audio track plays ominous voices overhead.

In Korean, the number six is pronounced yuk, which is another word for meat. “They say that people were taken to the Sixth Bureau to be butchered like meat,” said Yang Seung-phil, a former manager at the hostel.

During the Korean War, South Korea was virtually leveled, and like a dragon rising from a ditch — an old Korean saying — a new nation was born. Observing what remains and what has fallen to ruin or been destroyed — or revived — is an exercise in understanding the country’s national identity.

“History is not without words; it is only when it is not spoken of that it is silent,” reads a brochure for the Sixth Bureau Building memorial site.

In Seoul, gleaming high-rise buildings tower over meticulously maintained ruins dating to the Joseon Dynasty, whose rulers governed Korea from the 14th century until the beginning of the 20th, when the country became a Japanese protectorate. The occupation lasted for more than three decades before Japan surrendered to the Allies at the end of World War II.

Korea was then rocked by a civil war between the communists in the North and the U.N.-backed forces in the South. The violence left the peninsula cleaved in half.

In the South, national monuments burned and strafed during the Korean War were rebuilt with precision and fidelity as symbols of national pride. But remnants of Japanese colonial rule were deliberately demolished in South Korea as recently as the 1990s.

The K.C.I.A. headquarters, however, where screams once echoed through hallways, has been allowed to stand. Some see it as a necessary reminder of the country’s flirtation with autocracy, others say it represents a bitter chapter that many would rather forget.

When Kang Yong-joo was a medical student in his early 20s, he was accused of distributing pro-communist material and tortured at Namsan in 1985. He said many survivors of the K.C.I.A. suffer from post-traumatic stress and repressed memories. It’s not uncommon for victims to “feel sick when they see Namsan,” said Mr. Kang, who has seldom revisited the mountain.

The K.C.I.A. was once the most powerful institution in South Korea, established in 1961 with the help of the American government after a successful coup led by the South Korean dictator Park Chung-hee. A State Department official once described the agency as “a combination of the Gestapo and the Soviet K.G.B.” Its practices also included extortion, lobbying American lawmakers and intimidating Korean immigrants in the United States.

The headquarters on Namsan were built in 1973. That same year, Choe Jong-gil, a law professor at Seoul National University, was tortured to death during interrogation. Some historians believe he was the first victim to be killed at the site. The right-wing government maintained that he caused his own death by jumping from a window.

“In the case of Namsan, it’s really kind of promiscuous the way the past and the present interact,” said Bruce Cumings, a historian and the author of “Korea’s Place In the Sun.” “The largest Japanese Shinto shrine during the colonial period was on Namsan. It got destroyed immediately after liberation in 1945.”

In its place, the Korean government built a large statue of An Jung-geun, the Korean nationalist who assassinated Japanese prime minister Ito Hirobumi in 1909. “So you had a replacement of a Japanese symbol with one of the biggest symbols of Korean resistance to the Japanese,” Mr. Cumings said.

The K.C.I.A. underwent a series of changes after South Korea’s pro-democracy movement and is now called the National Intelligence Service.

At the hostel, staff members are quick to smile and greet guests, and the unfussy rooms with bare walls and bunk beds make it difficult to imagine the suffering that once took place in these same quarters. Most visitors are blissfully unaware of the history.

The local government, which owns and operates the hostel in collaboration with a nonprofit, took pains to make the building a welcoming place for young people, beginning in 2006. During the pandemic, the building was turned into a treatment center. More recently, renovations were made to modernize the rooms as tourists flocked back to Seoul.

The basement is now run by the Seoul Emergency Operations Center and is strictly off limits to civilians.

Han Hong-gu, a history professor at Sungkonghoe University in Seoul, said he wants the hostel to be turned into a museum dedicated to the democratization of South Korea. In 2009, when the Seoul City Government considered destroying the former K.C.I.A. buildings on Namsan, Mr. Han, 63, organized a campaign opposing the plan.

“Some buildings should be kept to teach the later generations lessons of history,” he said. “A site of dark history should be preserved.”

Near the entrance of the hostel, a nondescript mailbox invites visitors to write letters reflecting on human rights. Hwang Eui-sun, who used to work at the hostel as president of the nonprofit, said the mailbox didn’t have any real function these days, but was there as a “symbol of remembrance for the road to democracy.”

Cobwebs have gathered around it.

Jin Yu Young and Choe Sang-Hun contributed reporting from Seoul.