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Seoul was built to defend against attack, but its fortifications have diminished in comparison with North Korea's rise in weapons sophistication

A military installation atop Namsan Mountain in Seoul, built in case of attack from North Korea.

If you don't know what you're looking for, the small row of apartment buildings on the southern banks of the Han River are unremarkable mid-rise boxes, built like so many others in the South Korean capital.

But cast your eye up about halfway, and something odd appears: the stairwell windows on the sixth floor are much smaller than those on other storeys. Look closer and it becomes clear they are fortified as well, tucked into thick walls that overlook a river that, if war breaks out on the Korean peninsula, would provide a key natural barrier to the advance of troops from the north.

Those windows are, in fact, gun ports. Inside, the floors have been reinforced in case one day they need to support soldiers and heavy firearms positioned here to repel invading forces. It is a military installation inside an apartment building, unknown even to most of its residents.

If war breaks out, "you're going to be shooting at the boats, at people trying to cross by bridge," said Park Cheol-soo, an architecture scholar at the University of Seoul who has devoted a chapter in an upcoming book to the city's civilian fortifications.

"They're to stall and to buy time for the American forces to come."

Modern-day Seoul is a glass-walled metropolis, its skyline punctured by iconic buildings, its streets open and pulsing with life.

But as it prepares to host U.S. President Donald Trump for a visit that will focus on the North Korean threat, Seoul will do so as a city that has for centuries been built as a fortress.

Mr. Trump's motorcade will pass by planters that double as pillboxes, subways designed as fallout shelters, skyscrapers equipped with machine gun nests and at least one key bridge that can be quickly shattered if necessary to halt invading forces.

Today, with tensions again high, the city's defence mechanisms have begun to once again attract attention, with several bunkers being transformed into art and tourism spaces, including one that historians believe was built decades ago to keep the country's president safe.

A former bunker beneath Yeouido Island in Seoul has been transformed into an art exhibit.

But the city's architecture of defence goes far beyond an underground VIP safe room – even today, Seoul remains a city built to defend against attack although, like moats around a castle, their value has diminished with nuclear-armed North Korea's rapid rise in weapons sophistication.

South Korea remains technically at war with North Korea after hostilities with the north ended in a 1953 armistice rather than a peace treaty.

Seoul, the political and economic heart of the country, is particularly vulnerable. It lies within artillery range of North Korean weapons.

During the Korean War, North Korean troops for a time captured the city – and, in the years that followed, continued to stir fear in the capital. In 1968, North Korean commandos staged an unsuccessful raid on the Blue House, the country's presidential palace. Six years later, a North Korean sympathizer attempted to assassinate then-president Park Chung-hee. The attack failed, but the president's wife, Yuk Young-soo, was killed.

By that time, Seoul was already in the midst of a broad campaign to bolster its defences.

After the 1968 raid, "there was a resurgence in demand for the fortification of the city," said Prof. Park.

That notion "has been absolutely consistent and has persisted over the years – this idea of Seoul the fortress, the city as one huge defence system."

It was a highly public campaign, with companies placing slogans around building sites that read "Seoul Fights While Constructing." The broader corporate community got on board, too: in 1969, one energy drink manufacturer promised in an ad: "Drink this and you will be energized to fight while you build."

South Korea remains technically at war with North Korea after hostilities with the north ended in an 1953 armistice rather than a peace treaty.

"It was a total call to arms for the entire city, for the entire country," said Prof. Park.

What made it unique was an intentional blending of military and civilian functions.

Some of the earliest innovations included thick-walled concrete flower planters that could, in case of attack, double as pillboxes to shield soldiers in a shootout. Such planters remain scattered across Seoul, including around important sites such as the Blue House.

Behind the Blue House, Namsan Mountain was turned into a joint military and tourism site, with bunkers, military outposts – and a pagoda. The pagoda remains open today, offering clear views of a pair of military installations, one with an enormous twin-barrel weapon pointing at the sky.

Such visibility was by design, Prof. Park believes.

"By opening these to the public, the public were constantly reminded of, and put in close proximity with, the fact that we're at war with North Korea," he said.

By 1970, the local building code had been amended to include defence considerations, requiring high-rise apartments to devote a percentage of floor area to basement bunkers. Many 12-storey buildings were built with large underground protective spaces that remain important refuge locations scattered across the city.

Subways in Seoul double shelters in case of attack from North Korea, and are equipped with emergency supplies.

The city's subway system was similarly built with disaster in mind.

"A lot of the stations double as nuclear shelters," said Ryu Sun-yeol, deputy station master at the Jongno Service and Safety Center.

Platforms are equipped with gas masks, oxygen masks and other equipment.

A series of apartment complexes erected in the 1970s were designed for quick demolition in case of an attack to create rubble fields that could slow advancing troops.

None of this was in keeping with the original plans for Seoul, whose 14th-century founders envisioned "a city in which people could live their lives without experiencing the brutality of war – a city without war altogether," said Ahn Chang-mo, a professor at Kyonggi University who specializes in architectural history.

"They thought they could sidestep the problem of war altogether through good diplomacy."

That innocence ended in the late 16th century, during the Japanese invasions of the Imjin war. Seoul began to build fortresses on peaks around the city – peaks like Namsan that remain important for modern defence.

By 1970, the local building code in Seoul had been amended to include defence considerations.

Over the years, the city itself was reconfigured for military purposes.

In the early 20th century, Japanese occupying forces razed homes to create a series of north-south roads that could serve as fire breaks in case of aerial bombardment.

In the 1970s, planners began to push residential development south of the Han River, using the river as a form of protection for residents who moved there.

In the decades that followed, defence officials have continued to closely collaborate with developers and planners in the construction of Seoul; many of those details remain military secrets, but the city has, throughout the modern era, more vigorously developed in a southerly direction, away from the northern threat.

Government buildings have deliberately split officials between different locations connected by what Prof. Ahn calls a "giant subterranean web of shelters."

What's clear, however, is that Seoul's defences, largely built against ground attacks, can only do so much against a North Korea armed with atomic weapons.

"A nuclear war is something we're not prepared for," said Prof. Ahn.

"We could, of course, change building regulations so that all buildings could withstand nuclear attack, but that would involve a terribly large amount of money. It is not realistic."

Seoul’s subway platforms are equipped with gas masks, oxygen masks and other equipment.

Even bunkers beneath apartment buildings, he said, are likely to provide no more radiation protection than hiding in a windowless bathroom or closet in case of nuclear attack.

For many, the city's bunkers have already become a mere historical artifact. In recent years, a pair of old bunkers have been opened to the public, including one in Yeouido Island, which is now the city's primary media and financial district but was once home to military facilities.

The bunker contains a large main hall that, today, has been transformed into an art exhibit. A side room appears to have been designed with VIPs in mind, including perhaps Mr. Park, the late general and president, who once stood above-ground near the spot at a military event.

Inside, couches have been reupholstered with a leopard-like print, similar to the soggy originals that were found when the secret location was accidentally discovered in 2005. It was first opened to the public a decade later.

For many, Seoul’s bunkers have already become a mere historical artifact.

"The fact that a place like this exists here, where I work, just serves as a reminder that we are under constant threat," said Jihee Lee, a television producer whose office is nearby.

But she and other visitors feel the threat has been exaggerated by outsiders.

"If Seoul becomes dangerous, the world becomes dangerous," said a retired professor who gave only his surname, Boo.

An attack on Seoul, he explained, would likely draw in China, the United States and its allies – including Canada – "and the world would end. It would be Armageddon."