A capital founded inside a ring of mountains.

01 Four mountains and a river

The four inner mountains are still there and can be picked out by eye from downtown. To the north, Bugaksan, 342 metres, the most imposing, with its bare rock crest. To the west, Inwangsan, 338 metres, recognisable by its rounded granite blocks. To the south, Namsan, 270 metres, today crowned by the communications tower that appears in every photograph. And to the east, the most modest, Naksan, at barely 125 metres.

Work on the new site began in October 1394 and the main palaces were finished by 1395. Gyeongbokgung, the principal palace, was placed at the foot of Bugaksan, so that the mountain sits literally behind the throne. It is the most explicit relationship between architecture and terrain visible anywhere in the city.

02 A wall that follows the ridgelines

The logical consequence of that siting was the wall. Between 1395 and 1396 Hanyangdoseong was raised, an enclosure of 18.6 kilometres and an average height of seven to eight metres, traced along exactly the ridgelines of those four mountains. No perimeter was drawn on flat ground: the wall was carried up onto the ridges that already closed the basin.

It served as the capital's wall for 514 years, from 1396 to 1910. Eight gates were opened between 1396 and 1398, of which six survive. There were major rebuilds in the mid-fifteenth century and another in 1704, the latter using large dressed stone blocks: walking the wall, the different masonry types are visible to the naked eye and let you date each stretch.

Much of the enclosure was demolished during the Japanese occupation, and today the wall does not form a continuous ring. The best-preserved stretch is the 2.3 kilometres climbing Bugaksan between the Changuimun and Sukjeongmun gates. The ensemble has been a South Korean Historic Site since 1963 and appears on UNESCO's World Heritage tentative list, though it is worth being precise: it is not inscribed.

03 A national park inside the city

Beyond the inner ring lies the massif that completes the idea. Bukhansan National Park covers 79.92 square kilometres, was designated on 2 April 1983 and lies partly within Seoul's administrative limits. It is not a weekend excursion to the outskirts: you get there by metro.

Its summits are bare granite and have genuine height: Baegundae, the highest, reaches 836.5 metres; Insubong, 810.5, with a face well known among climbers; and Mangyeongdae, 799.5. The park receives some five million visitors a year and holds around 1,300 plant and animal species. Trails are closed in rotation to allow the ground to regenerate.

It is often repeated that Bukhansan holds a world record for visitors per unit of area. The claim circulates everywhere, but it does not appear in the official record registries, so it is best treated as a very widely repeated assertion rather than a verified figure. What is checkable is striking enough: five million people a year climbing a granite massif that begins where the metro ends.

04 Twenty-six million around it

The city proper has some 9.6 million residents across 605.21 square kilometres, giving a density of 15,828 people per square kilometre. But the surprising figure is another one: Seoul is shrinking. It once passed ten million and has been losing population for years, while the metropolitan area grows around it.

That metropolitan area — Seoul, Incheon and Gyeonggi province — adds up to around 26 million people across some 12,685 square kilometres. It holds more than half the country's population in a single conurbation, and roughly half its economy. The mountain you see from downtown is, in practice, the edge of one of the most populated urban regions on the planet.

05 How to read it on the ground

On the ground, the city reads better looking up than looking sideways. From almost any downtown avenue you can see at least one of the four mountains closing the view, and that constant presence is what stops Seoul from feeling like a featureless plain of towers.

The river supplies the second scale. The Han crosses the city from east to west and splits it in two, with Namsan and the historic core to the north and the newer districts to the south. It is a wide channel, crossed by numerous bridges, and it works as the only open horizon in an otherwise densely built city.

If you have to pick a single walk to understand the whole scheme, climb the wall. Any stretch works, but the Inwangsan one is the most eloquent: from up there you see at once the fourteenth-century stone underfoot, the palace layout below and the towers behind, all inside the same bowl of mountains.